US #504 Color Varieties

Although these are nothing of much note in terms of condition and/or value, I was recently going through some Washington Head stamps and came upon these two examples:



Through process of elimination, these two stamps -- both perf. 11 -- would have to be Scott #504. However, there seems to be such a significant color difference, with the stamp on the left even looking as though it could be on bluish paper as compared to the example on the right. Is this normal?

I do note the Scott Catalog references blue, light blue and dark blue varieties, so are my stamps simply the "dark blue" and "blue" examples?

I would think it is almost certainly from the printing process wt1 as far as the bluish paper saga goes. I actually acquired some actual bluish paper stamps and they look almost nothing like you might be led to believe. They are almost a greyish color and could be easily overlooked.

These are 'normal' variations. Maybe one was left in the sunlight too long.