Once you learn how to use an editing program, you'll be able to edit your tile-sets and create variations to your pieces. Items like trees you can make a few different coloured ones, which is what I did. It's really a neat way to add a different look to your game as the most common item used in RPG Maker games are the trees, which is why I only use the single ones with a few colour styles to make them different.
I think you could learn to do that in a few days. I'm not sure how good it is, but GIMP is a free editing tool and you can put your tile set in there and make some edits. It's an advanced skill to have, just back everything up in your game first before playing with tile-sets so you can undo any mistakes.