Changing the color of the whitespace indicator in Visual Studio?

You can do that in Tools / Options and then Environment / Fonts And Colors and there is a setting for visible White Space :

I was searching on how to do the same on VS code and I ended up on that thread. I found out that you can change it on the settings.json file. This one you can open if you go on Preferences/Settings and search for Color Customizations for the Workbench.

On the json file accordingly you will need to add (if it's not already there) the next property:

    "workbench.colorCustomizations": {
       "editorWhitespace.foreground": "#1e6d9b"
    },

The color should be a hex code. In the example above I have "#1e6d9b" but feel free to replace it with your preference.

To customize for a specific theme only, use syntax like this, where Monokai is the name of the specific theme you want to override the value for:

"workbench.colorCustomizations": {
    "[Monokai]": {
        "editorWhitespace.foreground": "#1e6d9b"
    }
}

Complete reference on customizing a color theme: https://code.visualstudio.com/docs/getstarted/themes#_customizing-a-color-theme
and theme color properties:
https://code.visualstudio.com/api/references/theme-color