What is the hardest color for deer to see?

While deer have no trouble seeing short-wavelength colors like blue and green, their eyes are not well adapted to seeing long-wavelength colors, such as red and orange. This is what makes hunter-orange so useful in the field. Humans see it as a neon alert of another hunter's presence while deer see it as a muted yellowish-gray that blends in reasonably well in mixed woods.

https://www.fieldandstream.com/stories/hunting/deer-hunting/what-colors-can-deer-see