The grass color in the Lush Caves biome is the same as the river and most ocean biomes, which is a dull blue-ish green color that really doesn't look that lush or vibrant and doesn't blend well with the moss blocks. 

By looking at the code, the temperature and rainfall values of the lush caves are 0.5 and 0.5 respectively, maybe raising the temperature value would help to fix this issue.

Also, this seems to be a parity issue as in Bedrock Edition this biome has a more fitting grass color.