The incredible vanishing decal

Sounds like a case of translucency. You may have to make multiple decals and sandwich them. Put a white one down (or two) and then put your yellow one on top of them.

I'll have to try light gray, I don't have any way to print white decals on clear film. Always SOMETHING new to learn on your way to finishing a model ...

I've had the problem before, and, with no way of making my own decals, had to pop for a new kit to get another decal sheet. Of course this helped build up the stash, but some of the kits were as bad as the decals.

Well, after several attempts at "layering" decals, primarily various shades of light gray under the bright yellow serial numbers, I'm about ready to give up. It seems that no matter what color combination or printer setting I pick, the numbers all but disappear when put down on the dark olive drab rudders. Any other suggestions would be welcome.

Hmm, you may have to give consideration to making a mask and painting it on.

 Use the white decal paper not the clear. Youll have to cut it very carefully but it will help out.

I, too, have had the problem with inkjet printings in yellow and also light gray disappearing into the background, especially if it is a blue or OD. Multiple layers did not work. Agree with woodbeck3, use white and cut carefully. However, if using Word, you can set up a box around the yellow printing, send to the back so the text is in front, adjust the box color to match your background paint, and print. Cut close to the printing, install decal, touch up with the background color. This has worked several times for me. However, getting the box color to match can be a bear and will take several attempts to get it right or close. There was a web site I ran across that had color matches for microsoft colors to many FS and other colors, but do not have it at my fingertips right now. Am just starting to try their color matches so can not say if it is good or not. Oh yeah, use white decal paper for this procedure so the yellow has the white background to lay on.

What you might want to try is reprinting the same sheet several times.  My printer feeds very well when I feed the same sheet I printed before.  I have tested the method and the colors are darker, however, I haven't used the decal on a project yet.  I don't know how many reprints it would take to make the colors visable but every time you feed the sheet your taking a chance of a mis-feed or register problem.