You can change colour and font according to methods that is supported by the codec pack.
DirectVobSub
DirectVobSub configuration -> Main tab -> Text Settings -> click on "Arial". There you can choose the font, colors, and font size.
ffdshow video decoder
ffdshow video decoder configuration -> Subtitles -> Font. There you can choose the font, colors, and font size.
ffdshow video decoder configuration -> Subtitles -> VobSub. There you can adjust the size (scaling) of image based subtitles.
Internal subtitle renderer of Media Player Classic
MPC Options -> Subtitles -> Default Style. There you can choose the font, colors, and font size.