This is the true color, on a card proof, from my collection.
The USPOD had five printings of proofs which were sold to the public in the latter part of the 19th century. These were on both India paper and card. These sold as sets and mailed in specific envelopes designed to hold the proofs. While I do not have an example of the envelope for the Columbian set, here's an example of the envelope used for the first issue proofs, Scott 40-47. The same type of envelope was used for the Columbian series, with printing which would have identified it as such. Here's the envelope:
Comparatively, the designs are a match, so both are complete. The only listed essay for Scott 235 is for an essay which exhibited incomplete design of the horse's head and the four corners. This is ruled out.
There are no trial color proofs listed for this stamp, but there are a series of trial color proofs for the two cent and the four cent listed in Brazer's book on essays. There is a listing for dim dusky blue, which this might be. These, however were printed on thin hard card and you state your example is on thin white wove ungummed paper. That presents a problem.
Given that the color is supposed to be purple, what you likely have is a color changeling on India paper, Scott 235P3. If this is India paper, you should notice that it is very thin, and white. This type of paper was used particularly for proofs as it readily absorbed the ink, providing a clear impression of the stamp and color. These are also plate proofs as an entire plate would be made up and printed, as opposed to a die proof which used a single die to create the image.
It is possible that this is an unlisted trial color variety, which Brazer was unaware of. To determine which it is, you will need an opinion from an expertization service. The best one for this piece would probably be the Philatelic Foundation.
Before doing that, I would do as chipg recommended and contact James Lee. Get his opinion and if he feels it might be a trial color proof, then go with the certificate.
Hope this helps.