What has been said above - to get white numbers on a plain dark back ground the trick is to paint the area white, print the decal with white (or light) characters and a background to match. This simplifies the cutting out no end, and as part of the varnishing, the edges can be overpainted.
Using a colour laser (these are a lot cheaper than they used to be) you can get vinyl sticky back decal sheets. These work great, but need careful cutting, a 1:72 maple leaf for a corvette funnel is an absolute bitch. It also has a very finite thickness, so will never be flush like a water slide.