Finland #21 - Which Blue?

Admittedly I know nothing about the stamps of Finland. Just looked at the few I have again after a long time ignoring them.
Found this older number from the 1875-81 series, perf. 11, so I know it's not the later issue.
But I'm unclear about the colour. There are three blues listed in Scott's for this denomination. Ultra (#21), blue (#21a) and prussian blue (#21b).





I'm aware of the difficulties in identifying a colour on our computers (scanners, monitor settings etc.) so I've found a scan of a KGV prussian blue stamp for comparison.




Obviously the same problems arise comparing these two scans, but does anyone think my Fin could be prussian blue?
The value of 21b is much higher than the other two, so of course I'm curious.
What do you all think?

OK, no one wants to take a stab at it. Let's try this a different way.
Here's an image of a 21, 21a and 21b from ebay. The listing however doesn't say which is which.




Can anyone label these colours, left to right, now that we see them in context?
Mine looks like the one on the left, I think.

@jamesw... Judging the color from image is extremely misleading as each display behaves uniquely. What seems bright blue in my display, can appear dull blue or ultramarine in others. In order to identify colors accurately, each device (scanner, display etc) in the chain used to produce and display image should be calibrated properly - and it's unlikely to happen...

But let's give it a go the other way... Specialized Finnish catalog lists 30+ shades with know print dates/runs for this particular stamp alone. Based on cancellation (HELSINKI 1881) , Your stamp is likely one of the later (low value) prints.

Oh, and not to make things even more difficult... Perf 11 does not necessarily make it earlier issue (though it likely is)....These are complex series (and I'm nothing but a novice/intermediate with these).

Hi scb. Thanks for the response. As I stated in the initial post I am aware of the vagarities of scanning and displaying colour, and trying to judge them. That's why I thought I'd add some visual examples. That way I'd hoped to at least get an idea of what the possibilities were.
Unfortunately Scott's only lists the three colours (wouldn't it be great to be able to have a specialized catalogue for every country?). I did see the cancel was 1881 but the first issue was released (again, according to Scott's) from 1875 to 1881, so that's why I figured this (along with the perfs) was from that issue.
What do you think the three blues pictured in the ebay image above, are? I'm still inclined to think the first one is the same, or similar to mine.
I know it's wishful thinking that mine is the prussian blue, but hey, never hurts.
By the way, is that your stamp blog listed under your response? Looks great, I'll have to take a closer look when I can.

@jamesw... Yep. That's my blog :)

Re, the Finnish stamps on ebay pic... The first does have similar appearance to Yours. I'd say that it (and your copy) are what my specialized catalog refers as "greyish ultramarine, issued March 1881. The rest on the ebay pic would appear ultramarine-blue/blue, suggesting that they are from later Charta sigillata printing (which Scott categorizes as 1881/83,perf 12½ - which is simply so untrue).... All on the low end of catalog values. Just my opinion.

If looking for the expensive types, then I'd look for copies with postmark 1875 or early 1876. Specialized catalog defines the color in these as green-blue, azuere-blue or grey-blue, but each can have notable variation within the same print. Which is why You usually have to rely either on date of postmark, and other characteristics when trying to identify these correctly.

But Like said earlier, these are complex series with several books dedicated to them. And I'm by no means expert/specialized with these.

Thanks scb. Appreciate the info. Scott's obviously simplifies this.
I'm not looking for any of these, just stumbled onto this one, and hoped for a payout. Was going to auction it off here if it was worth anything. It will go back in the envelope with the rest, probably end up on ebay or at my stamp club.