Black ink mixed with white ink = gray
Black ink + dry print = gray (white paper showing through black ink)
Because the above is true, you have to look for signs of a dry print and in this case I think it is fairly easy. Look at the solid areas of ink and not at the overall stamp. A true gray ink would be seen in the solid areas, look at your stamp
Looks black to me in both stamps, hence this is a dry print.