Sand tiles?

I had a question about changing the color tiles on the draw tools menu. I was making a dungeon from scratch and accidently erased a square from an already made map that i loaded and got an idea. Is there a way to change the color chart tiles on the draw tools menu? How would you suggest changing the floor. My idea is to make a piece of sand in the corner in which i can hide coins and a small sand viper but so far ive just been able to put a plain yellow patch of floor tile. Can i make a grainy tile on my computers (paint) and then save the "sand floor tile" to the color chart?