Maybe it’s a problem with your font exporting settings.
I use BMFont, and when I export a font, in Export Options I go to Presets and select: WHITE TEXT WITH ALPHA.
If I want the font to have outline, I use: OUTLINED TEXT WITH ALPHA.
With those presets I never have any problems setting colors.
If you have no way of re-creating the fonts, then you should probably edit the yellow out of the PNG in photoshop or similar programs.
Hope that helps!