Choose Preferences > General > Editors > Text Editors > Quick Diff and change the Colors options.
Click Apply and then OK.
The colors you chose are used.
Quit Studio and re-launch. The colors have returned to the previous settings (unlike, say, the settings in Studio > Themes, which retain the new settings).
I note this in Studio 4.6.0, which is not a choice in the Affects Version/s dropdown. It is my recollection that this also appeared in earlier versions of Studio.