if you remove the back chrome cover "beauty plate" (3 screws I think but the gauge needs to be off the bike), you can access the bulb for the gauge. It is a rubber thing and it just pulls out the back. If you are going through the trouble of replacing it and you want a brighter light and color, go get an LED colored replacement for those lights (most good autoparts carry them now or you can get them from online).
if you look on ebay there are always guys selling white face kits. Or ytou can search the cb1100f.net board and cb900c.com for what those guys used.