If you want to know how to change part of your image, without disturbing the rest of it? There are an assortment of different ways to do this. Check out this article for a few methods.
Layers are a great way to separate different parts of your drawing.
For example, if you are drawing a face, you could break it down into different components. Create a layer for the eyes, another for the nose, mouth, face, hair, and ears. Then on other layers, you could add the coloring.
For example, you want to change the very center of this image.
Zoom into the center, then double-tap your eraser to access its brush properties and change the size of the eraser. Now, erase the area.
Once it's erased, you can continue.
Use Lock Transparency to effect only the pixels with pigment. So, if you want to change a certain part, you can find the layer its on, then tap to lock it .
Now, if you Flood Fill or paint, the new color will only appear where there were colored pixels
Use Flood Fill to fill an enclosed area with a color.
For example, you want to change the color of the eyes. Select Flood Fill, adjust the Tolerance, then select a color and tap the area(s) you want to affect.