what color is the human eye able to best distinguish shades of? For
instance, if there was a dark-colored needle, in a light-colored
haystack, what color (for both) would make it easiest to find?

If I understand your question correctly,

Human vision is most sensitive on average to approximately 550 nm or
light, or yellow.

But, there is the issue of contrast as well.

Yellow/black is the most visible color combination, which is why a lot
of road and warning signs are of this color combination.

So, a yellow needle in a black haystack would probably be ideal.