So starting off here are some pictures for reference, Kano has had a lot of looks throughout the franchise, so feel free to deviate. I'm mostly going off the first picture here and his more modern appearances.
On the figure: I chose this Nuke over the other because while Kano goes shirtless more often than not, the other figure has like two machine guns on its back, so is better for the actual Nuke design (Plus the belt is going the opposite direction which bothers me personally).
Step One: Firstly, you'll want to put a light coat of black over the body armor/vest (if you're anything like me you'll need to for when you inevitably make too big of a splotch and you need to cover it up). Also, as the leader of the international criminal empire Black Dragon, black just suits him better.
Next, you'll want to paint his chest loop diagonally across his chest (this can be either silver/lead or leather brown), starting at his shoulder strap. In the center of his chest paint a metallic circle where his cybernetic heart will go. There are little squares on his armor you can use to make sure everything is roughly centered. Also make sure to dollop the paint on a little more, so it's clear the strap is being worn over the vest.
Lastly use bright red to add a nice solid circle in the center of the chestpiece, again you can dollop if you want it more orblike. You can also add the additional brown strap if you like, I did this at the end.
Step Two: Use a light skin tone and cover up his face, being careful not to paint over his teeth, but everything else you'll be going over again. I had to mix with white and a little yellow (matching the tone was probably the hardest part for me, so don't worry about getting it perfect).
Then you'll do his hair, just paint it black then work along the sides for his muttonchops and add stache. You can go full beard if you want (he's worn both), but it'll be tougher to paint around his knife.
Use the same metallic color for his eyeplate using pictures as reference and dot the eye with the same light red.
Add a white dot for the other eye, a smaller brown dot (I guessed on natural eye color here), and an angry black eyebrow.
I also painted his grenade black, this is again optional.
Voilà, you've painted Kano. As I said there are a lot of iterations of him, including no hair, no facial hair, rectangle chest piece, white robes, etc. Remember this is your figure and you can paint whatever you want.
He also has some sick arm tattoos, but I'm not feeling up to do those today. Another thing to note, his boots are almost always black, that I will do soon.