Those 1x1 pieces are going to make things very tedious for you. Just do a single terrain piece of the size you need. Then paint the whole thing your base color. After you've done that, you can start adding paint layers on top of that, and they won't look so jagged.
You should notice that each of those terrain pieces have their own paint layers. That means that you have to redo the work for each one, which is annoying and suboptimal.
Also, the order of the paint layers matter. When you get to adding more, layers will cover those beneath them. So just make your base paint that bottom layer, and you can forget about it. For example, in a map I'm working on now, my paint layers for terrain are something like this:
1. Rubble
2. Slate Rocks
3. Wet Sand
4. Smooth Sand
5. Dry Soil
6. Slate Base
Does that make more sense? It's hard to explain it in text without making an image or a video explanation.
