Humans have trichromatic vision. Deer, Elk (wapiti) and some other animals have dichromatic vision. It's not that deer and Elk can't "see" oranges and reds, they just perceive them differently.
Here is an explanation from Polk County Iowa
Deer Color Blindness
Are deer are color-blind? Is this why hunters can wear bright orange
clothing and the deer don’t seem to notice?
Yes, deer have a form of color blindness. All mammals have a retina
located in the back of the eye that consists of two types of light
sensitive cells called rods and cones. Rods function in the absence,
or near absence, of light and permit vision in darkness. Cones
function in full light and permit daytime and color vision. Deer have
more rods (nighttime cells) and fewer cones (daytime and color cells)
than humans. Therefore, deer have better nighttime vision than humans
but poorer daytime and color vision. Deer lack red cones so they can't
distinguish between green, yellow, orange, red or brown. All of these
colors appear as shades of yellow. So the blaze orange color hunters
wear appears yellow, as does almost everything else to a deer.
Elk have similar vision as deer. According to this site,
When it comes to sight, elk aren’t endowed with precise vision. In
fact, they see around the human equivalence of 20-60. Additionally,
they don’t see the full color spectrum humans do. Elk vision is
dichromatic, which means their world is seen in two colors, not
trichromatic like our vision. There is a whole science to ungulate
vision, but to keep it simple, just know this. Elk don’t have a red
cone like humans, so the upper end of the color spectrum appears
yellowish to elk. They aren’t color blind, but they don’t see color
the way we do.
As a bonus, it's speculated that deer may have the ability to perceived ultraviolet light better than humans.
In August 1992, a group of leading deer researchers and vision
scientists gathered at the University of Georgia to conduct this
Our study confirmed that deer possess two (rather than three as in
humans) types of cone photopigments allowing limited color vision. The
cone photopigment deer lack is the “red” cone, or the one sensitive to
long wavelength colors such as red and orange. These colors aren’t
invisible to deer, but rather are perceived differently. Deer are
essentially red-green colorblind like some humans. Their color vision
is limited to the short (blue) and middle (green) wavelength colors.
As a result, deer likely can distinguish blue from red, but not green
from red, or orange from red. Therefore, it appears that hunters would
be equally suited wearing green, red, or orange clothing but
disadvantaged wearing blue.
The results regarding the UV capabilities of deer were equally
fascinating. Our results confirmed that, unlike humans, deer lack a UV
filter in their eye. In humans, this filter blocks about 99 percent
of damaging UV light from entering the eye. It also functions much
like a pair of yellow shooting glasses and allows us to focus more
sharply on fine detail. The trade-off is a loss of sensitivity to
short wavelength colors, especially in the UV spectrum. Because deer
do not have a UV filter, they see much better in the UV spectrum but
lack the ability to see fine detail. This helps explain why deer often
move their head from side to side when they encounter a hunter.