Replacing a ballast of the same type (3 lamp with 3 lamp, parallel with parallel) is fairly simple. The wiring connections and colors match up.
In this example, a fixture with a 3 lamp parallel ballast is replaced with a 4 lamp parallel ballast. The colors of the old and new ballasts do not match. This replacement can be difficult to do unless a person totally understands how this wiring works.
A 3 lamp parallel ballast can be replaced with a 4 lamp parallel ballast. The two yellow wires on a 4 lamp ballast are connected together inside the ballast.
Lamp Parallel Ballast