Unfortunately, I tried MiscData and it didn't change anything. If anyone else has managed to successfully change the colour with MiscData, it would be interesting to know. It looks like I'll have to use a custom floating text system for displaying bounty. Thanks all for your suggestions.
EDIT: I have just found a script by Vexorian that does what I want (after a few aesthetic tweaks). I'll link to it
here for anyone else who might want to do this.