I've had some success on light-colored lettering by reverse-imaging it (body color over white) and applying it over the lettering color.
It takes a lot of experimentation. But it's been rather successful on some basic stuff. Probably my best current example is the circus stock car I just finished. Decals are laser printed red over white (clear) and applied over a white-painted backing, or using white laser decal paper.
http://www.randgust.com/Cirstock04.jpgThe white lettering on the Climax tanks is reverse-image, too, using this method.
http://www.randgust.com/Clim130201_right_side_hires.jpgThe way I did it was to do a test sheet of identified red shades (by Windows color number) on a test block and apply the test block over a piece of white styrene. I just do it in Excel. Then actually paint next to them and determine which one was closest. It looks a LOT darker on the decal sheet than it comes out on the model because of the transparency issue. The final decal is applied over the lettering color. But, once you figure out what printed color matches a body paint color, you never have to do this again.
I've done this over yellow, too, for an LEF&C herald. I converted one into a JPG and then opened up the letters a bit for a better rendition using Photoshop.
This also works printing it on the Micro-Mark white laser decal paper; body color over a white decal. I haven't tried printing light yellows on a white decal sheet but I'm convinced it would work well too.
You have to really make the lettering font strokes bolder (wider) than you think for this to work, make the lettering color overly distinct, and be willing to tinker. But it works. The trick is to accept that you can't match body color by eye, you pretty much have to make a test color chart into a decal and go with what looks good on the final, not what looks good on the screen.
In a way, this is a stupid way to do this, if I had an ALPS or even any of my friends left that still had one, I'd do it that way. But I will say that with the quality of current laser color printers printing over laser decal paper, it's not the least bit pixelated. It's sharp. When I did have ALPS printed custom decals, they weren't this sharp.
I've done it on inkjet paper too, but if you're hitting it with Solvaset, no amount of decal bonder will stop the Solvaset from coming through the BACK of the decal and starting to slightly dissolve the ink. So I've decided that it's laser or nothing, the paper is better (inkjet was rubbery, this stuff is almost as good as Microscale), it settles down better, and you can beat it to pieces with Solvaset and it won't run one little bit. I've done some Cole circus wagons of my own design that I had to hit over and over and over to get the decals tight, and while nothing else went the way I expected, the lettering never ran.
