Modifying the color of grep
You can change the highlight color of grep by using an environment variable, GREP_COLORS, which you can set like this:
export GREP_COLORS='ms=01;31'
Numeric options
The numbers can style text, change the foreground color or the background color, or change fonts.
The starting conditions for all GREP_COLORS options are the terminal's default text style, font, and colors. Resetting any of these will revert to the terminal's defaults, not any of grep's defaults.
Legend
ᴀ;ʙ;…—;separates numeric options that you want to combine (e.g., bold yellow-on-black text combines options1,33, and40into1;33;40)+ᴇꜰꜰᴇᴄᴛ— ᴇꜰꜰᴇᴄᴛ gets turned on when you use that numeric option-ᴇꜰꜰᴇᴄᴛ— ᴇꜰꜰᴇᴄᴛ gets turned off when you use that numeric optioncolor,green,cyan, andgrey— in the "Text styling" section, these refer to the foreground colorcolors— in the "Text styling" section, this refers to both the foreground color and the background colorthis color— in the "Foreground colors" section, this refers to the foreground color; in the "Background colors" section, this refers to the background color
Text styling
An empty string or 0 resets all text styling and resets both colors to the defaults but does not reset the font to the default.
┏━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━┓
┃ ### ┃ GNOME Terminal ┃ xterm ┃ non-GUI TTY ┃
┡━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━┩
│ │ «reset style+colors» │ «reset style+colors» │ «reset style+colors» │
│ 0 │ «reset style+colors» │ «reset style+colors» │ «reset style+colors» │
├─────┼─────────────────────────┼─────────────────────────┼───────────────────────┤
│ 1 │ +bold, +brighter color │ +bold, +brighter color │ +brighter color, │
│ │ │ │ -forced grey │
│ 2 │ +fainter color │ +fainter color │ +forced grey │
│ 3 │ +italic │ +italic │ +forced green │
│ │ │ │ ● overrides 2 and 4 │
│ 4 │ +underline │ +underline │ +forced cyan │
│ │ │ │ ● overrides 2 │
│ 5 │ «no effect» │ +blink │ «no effect» │
│ 7 │ +invert colors │ +invert colors │ +invert colors │
│ 8 │ +invisible │ +invisible │ «no effect» │
│ │ │ ● underline appears │ │
│ 9 │ +strikethrough │ +strikethrough │ «no effect» │
├─────┼─────────────────────────┤ ├───────────────────────┤
│ 21 │ -bold, -brighter color, │ +double underline │ -brighter color, │
│ │ -fainter color ├─────────────────────────┤ -forced grey │
│ 22 │ -bold, -brighter color, │ -bold, -brighter color, │ -brighter color, │
│ │ -fainter color │ -fainter color │ -forced grey │
│ 23 │ -italic │ -italic │ -forced green │
│ 24 │ -underline │ -underline, │ -forced cyan │
│ │ │ -double underline │ │
│ 25 │ «no effect» │ -blink │ «no effect» │
│ 27 │ -invert colors │ -invert colors │ -invert colors │
│ 28 │ -invisible │ -invisible │ «no effect» │
│ 29 │ -strikethrough │ -strikethrough │ «no effect» │
└─────┴─────────────────────────┴─────────────────────────┴───────────────────────┘
Foreground colors
┏━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━┓
┃ ### ┃ GNOME Terminal ┃ xterm ┃ non-GUI TTY ┃
┡━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━┩
│ 39 │ «reset this color» │ «reset this color» │ «reset this color» │
├─────┼─────────────────────────┼─────────────────────────┼───────────────────────┤
│ 30 │ very dark grey │ black │ black │
│ 31 │ dull red │ red │ light red │
│ 32 │ dull green │ light green │ light green │
│ 33 │ dull yellow │ yellow │ yellow │
│ 34 │ greyish blue │ dark blue │ sky blue │
│ 35 │ dull purple │ purple │ purple │
│ 36 │ teal │ cyan │ cyan │
│ 37 │ light grey │ light grey │ light grey │
├─────┼─────────────────────────┼─────────────────────────┼───────────────────────┤
│ 90 │ dark grey │ dull grey │ dull grey │
│ 91 │ red │ bright red │ bright red │
│ 92 │ lime green │ bright green │ bright green │
│ 93 │ yellow │ bright yellow │ pure yellow │
│ 94 │ light greyish blue │ dull blue │ deep blue │
│ 95 │ light purple │ magenta │ magenta │
│ 96 │ cyan │ bright cyan │ bright cyan │
│ 97 │ off white │ white │ white │
├─────┴──────┬──────────────────┴─────────────────────────┴───────────────────────┤
│ 38;2;ʀ;ɢ;ʙ │ replace ʀ, ɢ, and ʙ with RGB values from 0 to 255 │
│ │ for closest supported color (non-GUI TTY has only 16 colors!) │
│ 38;5;ɴ │ replace ɴ with value from 256-color chart below │
│ │ for closest supported color (non-GUI TTY has only 16 colors!) │
└────────────┴────────────────────────────────────────────────────────────────────┘
Background colors
Note that the non-GUI TTY doesn't provide a brighter background color series.
┏━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━┓
┃ ### ┃ GNOME Terminal ┃ xterm ┃ non-GUI TTY ┃
┡━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━┩
│ 49 │ «reset this color» │ «reset this color» │ «reset this color» │
├─────┼─────────────────────────┼─────────────────────────┼───────────────────────┤
│ 40 │ very dark grey │ black │ black │
│ 41 │ dull red │ red │ light red │
│ 42 │ dull green │ light green │ light green │
│ 43 │ dull yellow │ yellow │ yellow │
│ 44 │ greyish blue │ dark blue │ sky blue │
│ 45 │ dull purple │ purple │ purple │
│ 46 │ teal │ cyan │ cyan │
│ 47 │ light grey │ light grey │ light grey │
├─────┼─────────────────────────┼─────────────────────────┼╶╴╶╴╶╴╶╴╶╴╶╴╶╴╶╴╶╴╶╴╶╴╶┤
│ 100 │ dark grey │ dull grey │ black │
│ 101 │ red │ bright red │ light red │
│ 102 │ lime green │ bright green │ light green │
│ 103 │ yellow │ bright yellow │ yellow │
│ 104 │ light greyish blue │ dull blue │ sky blue │
│ 105 │ light purple │ magenta │ purple │
│ 106 │ cyan │ bright cyan │ cyan │
│ 107 │ off white │ white │ light grey │
├─────┴──────┬──────────────────┴─────────────────────────┴───────────────────────┤
│ 48;2;ʀ;ɢ;ʙ │ replace ʀ, ɢ, and ʙ with RGB values from 0 to 255 │
│ │ for closest supported color (non-GUI TTY has only 8 colors!) │
│ 48;5;ɴ │ replace ɴ with value from 256-color chart below │
│ │ for closest supported color (non-GUI TTY has only 8 colors!) │
└────────────┴────────────────────────────────────────────────────────────────────┘
256-color chart
Above chart is a screenshot of content published in "ANSI escape code, 8-bit colors" on Wikipedia, authored by CMG Lee et al. on 24 February 2016 and later, and licensed by the authors under both the CC BY-SA 3.0 copyright license and the GFDL copyright license.
Colors from the 256-color chart above can be used as follows:
38;5;ɴ— replace ɴ with the value of a particular color in the chart above to change the foreground color to the closest color the terminal supports48;5;ɴ— replace ɴ with the value of a particular color in the chart above to change the background color to the closest color the terminal supports
For example, 38;5;214;48;5;30 will set the foreground to color 214 and the background to color 30, giving an orange-on-teal result on terminals that support it.
Note that not all terminals support all 256 colors, so it's important to realize that the chosen color might not be used. Only the supported color closest to the one chosen will be used.
For example, the non-GUI TTY only supports the basic 16 colors for foreground and the basic 8 colors for background, so the closest colors that end up being used may not be what you expect. As an example, the orange-on-teal selection above (38;5;214;48;5;30) shows as yellow-on-black in the non-GUI TTY, since those are the closest supported colors.
Fonts
10 is the default font. 11 through 20 are potential alternate fonts (20 usually means a font in the rare terminals that support it). Only 10 and 12 seem to exist by default in the non-GUI TTY, and none exist in GNOME Terminal or xterm.
grep doesn't properly switch fonts back before exiting, so run reset if you get stuck in an unwanted font after grep returns to the shell.
┏━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━┓
┃ ### ┃ GNOME Terminal ┃ xterm ┃ non-GUI TTY ┃
┡━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━┩
│ 10 │ «no effect» │ «no effect» │ «reset font» │
│ 11 │ «no effect» │ «no effect» │ -messed up font │
│ 12 │ «no effect» │ «no effect» │ +messed up font │
└─────┴─────────────────────────┴─────────────────────────┴───────────────────────┘
"Numeric options" section sources:
● "ANSI escape code, SGR (Select Graphic Rendition) parameters" on Wikipedia
● Experimental verification on GNOME Terminal,xterm, and non-GUI-mode TTY on Ubuntu 16.04
The default value of GREP_COLORS is 'ms=01;31:mc=01;31:sl=:cx=:fn=35:ln=32:bn=32:se=36'
The meaning of every element accepted on GREP_COLORS can be checked at GNU.org's manual page.
For completion, and as pointed out by @damadam, you need to add the export to your .bashrc in order to save the changes.
Related:
Multicolored Grep
Use different colors for every another grep
In addition of the answer from @M. Becerra, don't forget to add this export inside ~/.bashrc. You can do it like that:
nano ~/.bashrc
and type now your export:
export GREP_COLORS='ms=01;33:mc=01;31:sl=:cx=:fn=35:ln=32:bn=32:se=36'
This sets grep results to yellow, and I don't need to redo the export after restart terminal or the computer.