i print twice, once on translucent and once on clear and mount one over another.
There should be a backlit film setting which puts down a load more ink.
In Onyx for my 25500 there's a Kodak Universal Backlit media.
HP 2600? A Color Laserjet? Or is this the HP 26500? And if you're printing on clear for a backlit, yes, it will be washed out. Inks are very translucent. You can print on clear but need to mount to white acrylic. And there are other things you should do to print for backlit....search the forum for all the helpful hints.