I have an '84 Lotus Challenger that has a lavender pearl paint applied. It is in need of some touch up and I was curious as to what additive goes into paint to make it pearl. Anyone know?

I believe it is the same powder that actual pearls are made of --- calcium something............

I'd bet you can find some nail polish at Target that will be close.

Back in the old days, it was finely ground fish scales. These days it is synthetic pearl powders.
That's a pretty color. You ought to be able to get close by mixing some of the pearls from the hobby shop.
Testors makes several pearl colors. But you'll have to buy several bottles of paint and mix and test. And remember, wet paint always darkens just a tad when it dries. You might also look at fingernail polish and get very close.

You could mess around with these, either in Testors or nail polish, and probably get something very close, with enough persistance. But, as mentioned, you can also probably get close enough with an off-the-shelf nail polish. The touch-ups must be pretty small - your bike looks great.

Or a store that caters to teen / preteen girls.

Any pearl or metallic touch ups are difficult. For pearl I put down a coat of silver paint, let dry, followed by the color, in your case lavender. The silver base coat gives it opacity and some pearlescense.

Lots of good advice here recommending nail polish and hobby paints. The pearl finishes I've done consist of a solid (non-metallic) color coat with a pearl clearcoat over the top. As mentioned there are several pearl clears offered for hobbyists, each with a slightly different accent color that can change when viewed at different angles. One other material used in some pearl clears is ground Mica.
The finish in these photos is a bit more course than a typical pearl, giving more like a metalflake finish when viewed up close. At a distance it looks more like what I would call a pearl. I included them to exaggerate the color/pearl clear effect.

Thanks to all for the advice/ideas. Lotus had some really different colors back in the day. I must admit I was a little like Henry Ford when it came to color but instead of black it was sliver. This color was quite a change for me but I have come to like it. The bike was a CL find and was destined for the scrap heap but I got it in time to save it. I will check the nail polish first as it would be the quickest and easiest route. Thanks again.

The auto shop down the street from my work has a wide selection of pearls and "metalflakes" that can be added to any transparent finish. I was using some of that on custom electric guitars I was making a few year ago. Bring your frame into a auto paint store like that one and they can give you some quick advice on how to use it. If you can find the right nail polish, however, that would be much easier.