I think this is a pity that we can't use every color for the terrain painting. For example, I chose the tropical map to build my park, I can"t use snow painting to make an ice part in my map. Too bad, I just can use the paintings related to the map I chose.
If they gave us this option, the performance of the game would dramatically decrease. From my understanding, to have all of the textures in a park means that they all have to be on top of eachother on the terrain or map. So, having let's sayaround 20-25 textures on the map means that the game and your computer then have to render al of those textures, which would greatly decrease performance.
I would love to see an environment editor where we could choose which textures we want to use in our parks. This way, we still get to use the textures we want, but it does not decrease performance as much. If you want to use ice on your map, you could stack the ice themed rocks ontop of eachother to make a mountain.