Perhaps one of the strongest and most powerful option that you can do while painting is to lock down a layer! In Photoshop, there are four ways to lock a layer down:
Layer lock – you can paint within the confines of the object shape
Paintbrush lock – you can’t paint on the layer but you can move the objects around
movement lock – you can’t move anything in the layer
Lock all – stops all editing on the layer!
In the screen shot, I’ve used the initial layer lock. As you can see, you can’t paint outside of the shape when this is turned on. As the artist, you will find the ability to preserve the shape of the object one of the most fundamental tools that you can ever use!
Learn this well and you won’t waste time worrying about distorting the shape while you work on shadows and lighting! For example, you will find that you will use this option a lot when adding detail to a specific shape such as armor pieces, panels, mechanical creations, etc.
The list goes on and on! It’s amazing just how much coloring options you can do once you confine everything into its own shapes.