The bug

Most biomes use the grass.png and foliage.png colormap textures for deciding biome-specific plant colors. However, swamps do not (MC-30445). Unexpectedly, in Java Edition, they do not have dedicated colormap textures of their own, and the color appears to be hardcoded. However, colormap textures for these blocks do, in fact, exist in Bedrock Edition, making this a parity issue.

These colormaps appear to have been added in the Nether Update, making this a valid parity issue.

How to fix

Instead of hardcoding the colours for swamps, introduce dedicated colormap textures for grass and foliage in this biome and make it use these instead. Attached are the files directly from Bedrock Edition, alongside versions of them with all unused pixels removed (MC-276842).