I have an ALPS printer that will print white, but they haven't been in production for quite some time. There is also some question as to how much longer they are going to be supported since they require special ink ribbons.
I'm thinking that one "might" try to print white decals on white paper by printing the areas around the white with a color that closely resembles the car color. In other words, suppose you want a decal of a white box to apear on the side of a black car. You simply lay out the white box then surround it with black. Print it then cut it out as close to the white as you feel like getting. I realize that this is oversimplistic, but I think you can get my drift. If you were doing complex lettering like reporting marks or dimensional data, you would simply cut around the entire block and go from there. The challenge would be matching a printed decal to the paint on the side of the car that you were lettering but once you had that, you "should" be able to save it and use it again and again.
Just my thoughts... yes, it is a lot of fun to print your own decals.
dlm
It was the old ALPS printer, no longer available. They also printed in metallics. As for the options ...
One trick I've heard of is to paint the area to be decalled white, then reverse the white-printing area of the decal to make it clear so the white shows through. The trick is to make the background color of the decal match the base color of the item being decalled.
OK, that's clear as mud. Let's say you want a white decal on a boxcar red box car. Paint the area on the box car to be decalled white. On the decal, the printing should actually be clear, in a block of color that matches the box car red.
And I think that's the real trick ... getting the decal and car red colors to match. Best bet may to come as close as possible, and then weather as heavily as needed to hide the color difference.
Hope this helps!