That begs the question that presumably Thorn Do not know the mix.
Possibly not, but even under the best of circumstances it is very difficult to achieve "original" results witha touchup.
For one thing, the original "paint" is powdercoat that has been fused to cover the undercoat over steel. Liquid paint behaves differently, and can thin in the middle as the edges of the chip form a meniscus. Often, a chip will take out the undercoat as well, so one is left trying to touchup bare steel unless the undercoat is replaced first.
Some years ago, I wet-painted my tandem with Glasurit (liquid) paint, same brand then used by Mercedes. It came out great but over time, the inevitable nicks have formed from rock-strikes and such. It is surely difficult to get the spot-touchups to match the original even though I am using the same paint. The spots are either darker or form little lumps above the surface if the paint gets too thick. I've taken to sanding, masking, repriming and spot-spraying the areas in need and then cutting and leveling the paint with various grades of rubbing compound After to get a good match.
Thorn includes a small bottle of touchup paint with their new frames (or did when I got my Sherpa in 2011 and the Nomad in 2012). It tends to be a bit on the runny side and so pretty transparent. For my matte black frames, I've had better luck spraying a bit of matte black spray paint into a lid and then using a small 0000 brush to dot any chips after the carrier/solvent in the paint has evaporated a bit. The result looks nice and matches for color as well as tone.
If your frame is Tonka Yellow, then you may be in luck buying paint mixed to match the Tonka toys. There is an active collector's community devoted to restoring these childhood toys and so a demand has arisen for matching paint. Google "tonka yellow spray paint" (no quotes). One such source is here:
https://www.worthpoint.com/worthopedia/tonka-paint-original-color-yellow-173481094 I don't know how closely Thorn's Tonka Yellow matches the "real" thing, but it might get you close with minimum hassle. I've got a few of these childhood relics up in my attic I've been thinking of restoring to give to needy children. The fresh paint would make them much more appealing.
Best,
Dan.