I've seen on a pmmp server that the grass Color blue is how can i make this, i'll will make that in a plugin that all grass is blue
With the current MCPE version you can't specifically change the grass colour of grass, but you can change the biome of the grass, which will change the grass colour. Using a taiga biome would get close to your ask.
You can't actually change grass to crazy colours anymore, for some unknown reason Mojang removed that feature. The corresponding API methods were also removed in 3.0.0-ALPHA1 along with other breaking changes. Best you can do is set the biome colour.