As a dealer, let me recommend you do not pull the case apart to modify the color. As hiddenrun states, they are sealed well and are water *resistant* (3' for 30 min is the rating).
Dismantling the case will nullify your warranty.
I'm not here to sell you a pager, or talk you out of it. If you are set on changing the color, maybe a jar of modeling paint and a fine tooth brush might be a way to go.
I'm not sure what this could do to your warranty.
You might be able to hit Unication up via a dealer (if you don't have one I would be willing to help you out) and see if Unication would be willing to swap out the case for you. I haven't personally had any requests for a case swap, but I'm not saying it can or can't be done either.
Probably the simplest method, would be to get one of the nylon cases and keep the pager in there. The cases are black and would not only help it blend in, but also protect the pager. They are well worth the investment, IMHO.