Unfortunately you must have your map terrain set to 513x513 pixles, any bigger or smaller the terrain will not work when you open and use it in your map maker.
As for your other issue, you can only use three colours for three different terrains. Red: 255, 0, 0 Green: 0, 255, 0 and Blue 0, 225, 0
Those colour schemes will only work to make the terrain when you add this to your map. If you need any further help, don't hesitate to ask or send a PM~ :)