First thing we need to do is explode the part into individual faces. Click on the part that you want to change and go to Geometry, Explode.
The part is now exploded into individual faces as shown below. We’ll turn it back into a single part again after we’ve changed the colors.
All individual face colors on the part can now be changed as desired. Simply click on a face and within the properties go to color and select a color.
In most cases we don't want to leave the part in an exploded state because now all you can pick are faces. In order to put it back together as one part you select all the faces in the assembly tree and then go to geometry, Merge and this will combine it back into one part as you see below. Note: The file name is different from the original name. Simply right click and rename as desired.