Depends on how you are handling subtitles
I think the only way to do this is convert the subtitles to a .srt file (a text file with subtitles as raw text with timing information), then whatever player you use is responsible for rendering the subtitles in real time and depending on the player you have some control over font and color. If you just rip the subtitles directly then you are actually saving pre-rendered graphics which are displayed as overlays on top of the video so you have no control.
Converting to srt format isn't something that DVDFab does, but there are other tools that you can use in conjunction with DVDfab, but it's fiddly and time consuming because the pre-rendered graphics have to be passed through an OCR program to turn them into text and you may have to clean up the resulting srt file to fix problems with the imperfect OCR.
Big question is why do you want to change the colors?
