Although I like and support variety, I am not sure how color of the grass can be properly addressed for countless regions across the globe, even if we take seasonal variation out of the equation. To me it feels like it would be too much detail to properly represent all regions accurately in terms of grass color. The average color of the grass can change drastically even across very short distances. Since we are talking about North America in this case, you have both yellow and greener grass depending on region. If we were to pick one it would be positive discrimination as the rest of the grasslands on the same continent would have a different color. This is why I am more inclined towards choosing one generic color globally to prevent such comparisons. That's why I think the current color is fine as is.
I would like to remind that the Grasslands Biome in the game is representative of all of the following biomes:
Tropical & Subtropical Dry Forests, Mediterranean Forests, Woodlands and Scrub, Temperate Grasslands and Steppe, Flooded Grasslands & Savannas, Tropical & Subtropical Grasslands, Savannas and Shrublands; including Tree Savannas.
Since the "average color" of the grass differs greatly even within each biome, it would be near impossible to properly address the entire selection without positive discrimination.
That being said I do think the game lacks variety in its maps and I do want regional variety within the same biomes, particularly types of trees that spawn based on continent. This would be relatively easier to implement as all plant assets already have biome and continent tags in the game and the maps can procedurally generate these plants depending on location. Even temperature ranges can be relatively easy to address depending on location/latitude, but the color of the grass is a much more complex issue I'm afraid.