I think I found a rare one !!

[Slingshot]

I've been through thousands of C23 Airmail stamps over the years, and just went through a new batch of 400, and found what I'm pretty sure is a C23c which has ultramarine instead of blue on the design. A first for me, and one I have searched for for many years. This stamp is worth more used than mint, mints have a catalogue value of $150, while postally used are catalogued at $1,500.00. I compared several hundred to it in sunlight today, and none were the same shade, with the suspect ultra having almost a violet blue look. As a bonus, this one has a nice circular date cancel from Grand Central Station in New York, which is a very collected cancel.
In the pics it is the stamp in the middle, flanked by a light blue on the left and a dark blue on the right for comparison. I will have to send it off for an expert to confirm it's identy, but am 99% sure that is what I have.
Stamps are known as paper diamonds, and there are still many valuable stamps to be found in the "wild".

 

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[david680]

Be a nice find if you are correct, but if you have a color chart, you could try that before spending money on expertising.

 

[DeepseekerADS]

Be a nice find if you are correct, but if you have a color chart, you could try that before spending money on expertising.

I would tend to believe Slingshot is absolutely certain to have a color chart!

 

[Slingshot]

Yeah, got a Stanley Gibbons, and another one that I can't remember the brand. The color charts are basically about useless though, and I wouldn't recommend anybody purchasing one. They are as bad about shading varieties on themselves as stamps are. I collect shades, and some stamps produce a lot, I'll have a dozen or more of the same stamp, with all the shades. The US 2c of the 1902 issue has over 120 recognized shades, just for example. I enjoy showing the shades to all mint collectors, as they usually are satisfied with just one stamp, and ask them which one they have in their collection. Oh, well different strokes for different folks.