I have painted the interior of several cases including a Coolermaster HAF 932 case mod.
It is less expensive and easier to just just use a spray can of primer and a spray can of paint purchased at your local hardware store. I use Rustoleum brand. There is no need to used exotic paints. The interior of the case is not subject to deterioration.
Remember to remove all plastic parts from the interior. That includes the plastic tool free drive fasteners and tool free card fasteners. The paint will eat the plastic.