Probably something really simple, but in the registry there are 3 diff selectable numbers for the color yellow... #35 Yellow ... #53 Competition Yellow... and #53U Competition Yellow.   Are there actual differences in these colors?  If what are those differences? 

Actually only two yellows available thru the C4 years...the Yellow code 35, and the Competition Yellow code 53. The 53U simply means that IF the car was a two-tone, the 53 Yellow was the "upper" color. There were NO two-toned C4s in a yellow color from the factory, as far as I can see.
The "Competition" Yellow looks a little darker than the regular yellow on the cars I've seen....they also have a problem with the exhaust staining the rear bumper cover really bad. IT's hard to see with the nekkid eye sometimes, but you put on a good pair of sunglasses, and the stains stick out like a sore thumb! Lots of folks complained about it, too, which is why the code 53 yellow was only used a couple of years, then they brought it back for a couple of more years later in the series.

Thnx for the thorough response, Joel.   Agree the Comp Yellow is darker and generally brighter.  I marked mine in the regestry as #35, as it seems to be more like a sunburst yellow..  a bit paler than the Comp Yellow.   However, I've had to do some repaint work because the clear coat was coming off in multiple places, and actually went back with a bighter yellow... more like a comp yellow.  I like both, but that initial pale yellow had its own appeal, too.

It sounds like you're not sure what color your car is/was...the paint code for your car should be on the RPO label on one of the rear compartment doors. Mine is on the driver side compartment, but I have seen them on the other door, too. The paint color code(s) are usually at the very bottom of the list, and you might see a 35U and a 35L code on a solid colored car, since the upper and lower body are painted the same color. The yellow code for '86 was 35, so that is the only option you have.