Convert a RGB Color Value to a Hexadecimal String

Random ra = new Random();
int r, g, b;
r=ra.nextInt(255);
g=ra.nextInt(255);
b=ra.nextInt(255);
Color color = new Color(r,g,b);
String hex = Integer.toHexString(color.getRGB() & 0xffffff);
if (hex.length() < 6) {
    hex = "0" + hex;
}
hex = "#" + hex;

You can use

String hex = String.format("#%02x%02x%02x", r, g, b);  

Use capital X's if you want your resulting hex-digits to be capitalized (#FFFFFF vs. #ffffff).

A one liner but without String.format for all RGB colors:

Color your_color = new Color(128,128,128);

String hex = "#"+Integer.toHexString(your_color.getRGB()).substring(2);

You can add a .toUpperCase()if you want to switch to capital letters. Note, that this is valid (as asked in the question) for all RGB colors.

When you have ARGB colors you can use:

Color your_color = new Color(128,128,128,128);

String buf = Integer.toHexString(your_color.getRGB());
String hex = "#"+buf.substring(buf.length()-6);

A one liner is theoretically also possible but would require to call toHexString twice. I benchmarked the ARGB solution and compared it with String.format() and the toHexString solution has a much higher performance:

Convert a java.awt.Color to a 24-bit hexadecimal RGB representation even if alpha channel value is zero (e.g. 0000ff):

String.format("%06x", 0xFFFFFF & Color.BLUE.getRGB())

For uppercase (e.g. 0000FF) :

String.format("%06X", 0xFFFFFF & Color.BLUE.getRGB())