What you may be seeing on eBay are custom paints or at the least custom renumbering. With Microscale decal set
#87-1267 you can renumber your P42s. I know I've seen a few custom painted and lettered P42s on eBay, although they are fairly rare. But doing a quick search on eBay found this neat custom painted little guy:
Custom painted Amtrak switcher eBay auction
However I think you may not know that in addition to #10 and #55, there have been many other Walthers/Athearn phase V P42s released over the years.
These Walthers/Athearn phase V P42s go back several years. Two even have the relatively short lived high stripe phase V paint. These included #146, and #134.
The others, in the now standard low stripe phase V, include: #7, #10, #28, #47, #55, #116, #163, #182 (with Tommy Thompson lettering), #184, #204, and #207.
So if you want a factory painted phase V P42, you'll most likely have to get a Walthers/Athearn loco.
Walthers has the exclusive rights to Amtrak phase V (as well as phase IVb) paint in HO scale. So if someone wants to sell products with these paint schemes in the HO market, they'll have to either get a license from Walthers or partner with them on the release.
I also believe Walthers has the exclusive HO rights to the Surfliner and Amtrak California schemes. Could be a reason why we have not seen these paint schemes or even an attempt to manufacture the body style of these cars in the HO market. However, since both the schemes (
Surfliner)(
Amtrak California) are now appropriate for use on a select few Superliner I coaches, perhaps Walthers will release these schemes on their improved Superliner I coach in the future.