We discussed this when you came to help on asiekierka/FoamFix#5

In this comment, after your check, I confirmed that it occurs with only Forge, BetterFoliage, and a resource pack.

How should I go about determining if the texture pack is faultily designed, or if there is an issue occurring in BetterFoliage?

As it turns out, it's simple. The grass texture of Pixel Perfection isn't quite black & white enough, and BF thinks it should use the texture's natural color (slightly pink) instead of the biome color.

Raise the option "Short Grass / Saturation threshold" to 0.2 and it behaves correctly. It seems I set the default a bit too low - but too high a value runs the risk of the opposite issue happening (blocks being colorized when they shouldn't).