Well, even in humans, there's a gradient between having very light hair (all the way to nearly white) all the way down to black hair, and the in-between areas have a gradient between brown/blonde and red for a fairly wide range of hair colors, even if it's only about 1/6th of all possible colors on the color wheel.
So, basically, what's the size of the wedge you want to draw?
When talking about river snakes or desert snakes, though, it is worth mentioning that serpentine bodies are generally fairly decent at being amphibious - sea snakes are evolved almost entirely for swimming, and look eel-like, but a regular snake can be a fairly good swimmer, they just need to avoid cold areas.
Serpents and reptiles in general, however, have an advantage in deserts because their lower metabolisms (thanks to cold blood) mean they use less calories regulating body temperature, which means they eat less food, and they don't sweat and breathe slower while resting, so they respirate less air back into the atmosphere.
Serpents are also prodigious tree-climbers and rock-climbers, thanks to the good grip their bodies can give them. They can adapt well to vertical environments like mountains and cliffs as well as trees or rainforests, and can ford creeks and rivers well.
Basically, you can find a snake anywhere it's warm enough for a reptile to survive.
A sentient humanoid creature tends to become much more generalized and adaptable than their more specialized kin - apes are localized in jungles, but humans are everywhere. A serpentine race would probably be comfortable anywhere that doesn't freeze, and it makes a huge deal of sense for a desert creature to go underground - less overpowering sunlight, underground is the only place to find water, and in colder areas, it's even warmer underground at night than it is on the surface. If they took to being underground (like the snakemen in the game already), then they could spread almost anywhere, just so long as they avoid the surface during freezing temperatures. (Real snakes in temperate climates find underground burrows and hibernate away the winter.)
Anyway, I was mostly testing out the palette swaps...
I'm not sure I actually like the miko with that color of blue hair, myself, honestly.
I like the darker colors with the miko outfit, but the blue-gray doesn't go as well with the red. I think I like the dark purple most, honestly.
Here's a set of going around the whole color wheel. Some are "natural" colors, like faded blue/purples, while some are bright, bold blues for contrast. The middle far left, for example, is pure yellow compared to the sort of dirty blondes next to and above it.
The third on the top row is a "natural" hair color, dark auburn, while the top right looks almost the same, but is just a dark pure red.
The top left, by the way, is the base color I'm using now. (A peasant with the shading removed to make palette swapping easier.)
Taking the middle row from the third to the fifth, these are all yellow-greens that get darker and bluer/less yellow. Fifth is a desaturated darker green, while sixth is a pure green for contrast. Seventh and eighth on the row are a light and dark "moss green" that is blueish.
Working back from the bottom row, the eighth from the left (far right) is a dark green, while the seventh is a dark teal. There's a blue-green sixth, then fifth is pure blue, while the fourth is that color blue you said you liked in the miko, which has slight purple tints and is desaturated. Then there is a dark purple-blue in third, a light purple in second, and the bottom left is a red-purple "wine red" color.
So basically, pick how light-to-dark the scale and hair colors go, what percentage of the color wheel they take up, and how desaturated their hair and scales can be.