Likely Scott 233A - Pictures And Comparisons Included

I have a stamp book that was handed down to me from a relative and inside was a 4c Columbian marked as a possible 233a. I've included a picture of the 233a, a 1c I had on hand, taken in the same light as the 233a, and a shot from the Smithsonian study about various errors. What do you guys think?

From what I've seen in previous forum posts the color is pretty unmistakable, and in person this stamp is a much closer blue to the 1c than it is to pictures of normal 233's.

Yours is a shade of ultramarine, but here is a recent thread on the subject.

https://goscf.com/t/67064

Looks like a pretty regular Scott #233 to me. If anything, I think it looks like the original ink simply faded over time – perhaps it was exposed to the sun or other chemicals that altered the pigment. I am absolutely just guessing, in reality I have no idea. Just throwing my guess in and waiting to see what the experts on this forum have to say about your mystery. I hope someone is able to figured it out definitively for you.

craigk: Your link just goes back to this thread... Seems like you copied the wrong link by mistake?

As a neophyte with US material, I would have thought your image of Sc#233 and #233A were around the wrong way.
I would have adjudged Sc#233 to be "bright blue"
Interesting.

Yes, this is not a 233a, but I like how you are comparing against the 230, which is the right (wrong) ink that was used in printing the 233a error.
But it is just an Ultramarine shade.

pmann, the image from the Smithsonian study is misleading. The images of the four stamps are from different sources, so do not represent real comparisons. You can see this in the tone of the paper in each image; one is yellowish, one violet, one pale white and one grayish.

In the image below, both stamps coming from the Philatelic Foundation database, the left stamp is a certified #233a (probably the Bright Blue) and the right stamp is a certified #233, ultramarine shade. These images were taken about 4000 submittals apart but presumably utilizing the same equipment. I am cautious in that last statement because it seems that higher resolution images are made for rare items than for common material, so the blue error may be a photograph instead of a scan, for example. Anyway, note how much more similar the bright blue shade is to a strong ultra than appears to be so in the Smithsonian study: