Change color of balloons and text in Message.app
In OSX Yosemite colors are blue (for iMessages) and green (for SMS). There is no longer the ability to change these; Macs now look like iOS devices.

Hard way: (backup first!)

For Yosemite modify CSS here:

/System/Library/Messages/PlugIns/Balloons.transcriptstyle/Contents/Resources/balloons-modern.css


For El Captain it's now located in:

/System/Library/PrivateFrameworks/SocialUI.framework/Versions/A/Resources


But one needs to either disable rootless mode or to use a union file system mount to map the file over without rootless.

Hope this will help.