transparent decals

I had the same problem trying to print out bright yellow tail number decals for my Doolittle's Raiders B-25, they had oversized (non-standard) serial numbers on the tail. You can try a number of things. One may work for you, none did for me, I ended up buying aftermarket decal numbers that weren't exact but by then I was fed up:

• Adjust the ink setting on your inkjet printer to "photo" or "highest quality" or something similar. Check in your printer's Help Files to see what ink setting is recommended for printing pictures. That should give you a thicker coat of ink. Or you may have to buy a special ink cartridge designed for "photograph printing" use.
• You can try running the same sheet through the printer several times, if your printer has good registry the numbers should line up the same every time. You have to let the ink dry thoroughly between attempts and spray it with fixative before the second go around.
• Print your decals in the color you want, and then print out the same one in very light gray (since you can't print white with an inkjet printer. Put down the light gray decals first, allow to dry, then put the white ones down on top of it. That may give you enough "backing" for the white to show up.
• You could always paint the area under the decal white, light gray, etc., before you put the decal down, although that requires a certain amount of precision to make sure the painted area lines up with the finished decal.
• Say the heck with it, crack a cold one and start scouring the web for the aftermarket decals you need.

Or use the old standby and double up on the decals - print them out twice and then put one on top of another.

What luck... I came here to post this exact same question about transparent decals. I will try some your suggestions mfsob since as luck would have it.... a yellow set of markings. 

I just ran the a test print through twice and got perfect registry... but... the yellow is way off now.  Gonna try the gray decal underneath next.  I let you know how it goes.

Hope one of those methods works for you. Of course, what I REALLY want is a home laser printer so I don't have to worry about clear coating the decals after they're printed. What's eating beans and cornbread for another six months compared to the joy of fabricating crisp, ready to use decals?