Spain 50 M 1870 Ultra Marine Or Blue?

Whats the color of this stamp?

Looks ultramarine to me...

The 3 line cancellation, indicates "remainders"
Not a postmark.

Colour determination can have one leave their senses.
I still have problems with these two colours.
Decide yourself and stick with it.
The "Ultramarine" here is rather "deep ultramarine"
Flip your eyes between blue and ultramarine, blue can be a tad softer.

Identifying hues and shades of hues is for me one of the hardest parts of collecting. There are so many factors that can affect them over time. And that's when one has the stamp itself to study. Deciding from a scan is another order of difficulty. Also, like Rod, I have particular difficulty deciding between certain blues and certain ultras. I'd follow his advice: make your best guess and stick with it, at least until you acquire a few more examples for comparison.

StampHack,

Everyone looking at your post will see another shade. Just look at the top annd bottom of the picture you posted and you will see it is not uniform in its shade due to the lightfall. Also, my scanner will scan another shade than your scanner or phone. It is useless to ask what shade it is. The best way is to compare stamps. Also, what Stanley Gibbons calls ultramarine is not what Edifil, Michel or Yvert & Tellier call ultramarine. Furthermore, a photogravure stamp or engraved stamp will look different even if printed with the same ink, because the amount of white or other colours may differ and your brain mixes these with the blue shade.

The Stanley Gibbons colour key is how Stanley Gibbons names colours in certain catalogues. I found they become highly inconsistent at some time. You only have to look at the famous 1935 Silver Wedding Prussian blue and 10d Wilding they also call Prussian blue in the specialised and concise catalogues. The Silver Wedding Prussian blue was, in essence, turquoise. Collectors named it Prussian blue.

To me, your stamp looks more blue than ultramarine, but that says nothing.

NSK after looking at the stamp again and the picture I posted I have to agree with you. The stamp in person tends to be more on the blue grey side to me. I was talking to a family member who has taken color theory classes at university and they said pretty much what you stated. The perception of color from person to person and region to region and culture to culture can be quite different.

The Edifil Specialized, list 3 shades: ultramarine (normal stamp), light blue and deep blue. But, it's a 150 years old stamp!