This is one I'm documenting here because it's come up twice on the #freecad channel and because I keep forgetting it myself.
FreeCAD has a bunch of different ways to change the appearance of solids.
You could change the properties on the objects view tab:
You could use the appearance... tool from the object context menu
You could use the random color button from the object context menu
You could use the appearance... tool from the view menu (this is the same as the context menu. Why it's in 'view' and not 'edit' I'll never understand.
You could use the random color item from the view menu (see above)
There's at least one more that's different than all the rest. If you select the object and right click over the tree (not the 3D window, but the object tree) you get a menu item called set colors...
This one opens a task panel. With the task panel open, you can Ctrl-click individual faces in the object and then set different colors for them.