When textures start to turn into "rainbows" in the toolset, its often an indication that the module hasn't been saved often enough while working on it. its technically an issue with graphics cards, but is initiated by having to "remember" too many -back steps- by your CPU, especially on older computers, but not limited to them. Saving after it happens rarely helps. Save often. Personally, I save after every 2 to 3 dozen "things"*.
*Things = pasting something to an area, opening area or other object properties, drawing triggers and placing tiles.