If I insert an image in a Thunderbird eMail message, even selecting it in Windows Explorer, everything is fine with image colors. However,

If I have an image open in some photo editor, then select it (Ctrl+a) and paste it in a message, its colors are washed out (muted).

The same happens if I take a screenshot (print screen). Selecting this shot and pasting it in an eMail message results to muted colors.

Thunderbird (and Firefox) suffer from a color profile problem, a problem that can be fixed easily, by going to the Config Editor (for Thunderbird go to Tools > Options > General and scroll down the page). In Config editor write the word color “color” and change the entry called gfx.color_management.mode giving it the value of zero (0). Restart Thunderbird and enjoy the colors of pasted images!