While there are many ways to do this, I'll show you my preferred method. I work with multiple layers and masks because this will allow you the ability to make future changes more easily than just working on one layer.
If you don't know how to use Layers andjust as importantlyMasks, then look up some online tutorials and get familiar with them.
Step 1 - Make a new layer. Draw the circles on that new layer using the iCloud icon as a guide.
Step 2 - Make a Layer Mask. Use Image > Apply Image to "copy" the Layer information onto the Mask. Apply Image is a tremendously useful function. It would be a good idea to look it up and learn it as well. Please note that I am applying the Transparency and not the RGB or any part of the R, G, B.
Step 3 - Get rid of the "inside" circle lines and shapes. There are many ways to do this. Figure it out.
Step 4 - Fill your layer. I chose red. Now I have a nice red brain shape.