I have not ordered custom dry transfer, but I have made white decals, so will answer about that part of the question. There was an article in the January Fine Scale about making inkjet decals, including those with white in them.
Here is the short answer to the specific question. You use white decal paper. I keep both clear and white on hand. The problem is that the whole sheet of the paper has the white background, so you need to cut out the decal perfectly to prevent the white ghost border. One way to avoid this if you have a decent photo editing program is to set the "background" color to the color of the paint you will be putting the decal on. This works okay with primary colors, but with anything other than a black or white background you need to be pretty good at color matching with your inkjet printer.
A second problem is that after application of the decal, you can just see the white edge of the decal, and if it is a dark background, it will be noticeable. There are a couple of ways to handle this. One is that some magic markers can color the edge of the decal without affecting the release of the decal from the backing. Paint will not work, because it prevents release of the decal from the backing.
I usually "paint" the edge after application with a bit of the background paint, using the end of a toothpick. Slow and a lot of work, but it works.