It is possible to change the color by editing its texture, "rainTail.blp".
Below is a test map with one that I painted completely red. Here is a screenie of what it would look like:
You just need to edit the texture under
ReplaceableTextures\Weather\rainTail.blp
To export it, use any MPQ editor and look inside war3.mpq, then right-click the file and export it.
To convert it to TGA (so you can edit it in image editing programs), just use BLP Lab.
To edit it, you can use GIMP or Photoshop, and just change the hue/saturation.
Then you can convert it back with BLP Lab again.
Check our tools section for an MPQ editor and for BLP Lab. Use google to get GIMP. If it seems daunting, you can always make a thread in the requests section and give them this info. It is a pretty easy request.
Good luck!