Shown from behind: https://i.imgur.com/6MQm8cI.jpg
I'm working on these two fellas at the moment. I'm having a problem that I run into a bit when my miniatures are wearing collared jackets. I like to paint the skin first for some reason. As you can see, I was a bit too aggressive with the application of the Tanned Skin paint (Reaper MSP Series). The flesh-colored paint was accidentally applied to the inside of the jacket collar. I hate when I do that...Now it's going to be a pain trying to get the jacket leather color in there without touching the dude's face.
One thing I wanted to say is that I am not the biggest fan of the Reaper MSP Series paints, but I have been extremely pleased with their flesh-colored paints. They have several stages of darkness and you can buy the flesh color you want in single paint form or you can purchase the Triad pack which comes with the basic skin tone and then also the skin tone shadow & skin tone highlight paints so there is no guessing on what to use if you go that route.
Here is another example of Reaper MSP skin-colored paint. The photo isn't the best due to crappy lighting, but you'll see best on the right arm and parts of the chest. It's a beautiful shade.
They're your minis. Paint 'em how you want. Painted always looks better than bare plastic. Lots of folks around here prime white and just throw on a dark wash.
It's not Space Marines in 40k where someone's going to pop a vein because you want your purple space marines to use the red space marine rules.
Only real rule I'd have is to make sure you use paints that are meant for miniatures.
If you wanna take some of the guess work out, Reaper has color triad sets that are a shadow, base, and highlight color. https://www.amazon.com/Reaper-09715-Master-3-Piece-Medium/dp/B06XSB986N/
Vallejo has box sets of paints with examples and recipes for mixing. I bought this a few years back and quite like the set: https://www.amazon.com/Vallejo-Malefic-Flesh-Set-Paint/dp/B01D4VFCKY/
Also, you mentioned Citadel. If you play 40k or know someone that does, chances are they have bits they're not using. Especially heads with exposed faces - those might be good to experiment on.