About the green skin
I think it was already explained (don't remember wether in the comic or the comments) that Johnson consciously manipulated his melanochrome implant to turn himself green in order to "fit" in the Arena.
Also, Tyrnid carapace can be strong enough to deflect bolter fire and their "soft" tissue is similar to kevlar so what the hell is Selene wearing? Power Heels?
Ok, she's stacking "pain counters", that was a nice "furious charged kick", now she's fearless (and her combat style resembles Lelith's, soo...) and she's probably still under the effect of the "painbringer drug"......
About the green marine..... he made himself green so that he looked like an ORK in the arena (he even had a nice suit of trash armor to look like one), then he got the T.A.U.rminator suit with a H.E.avy flamer, a S.T.orm shield, and a 300lb stick of justice (T.H.under hammer)
(At same time, Kor'la got a nice experimental xv-8 series armed with lots of dakka..... guns)
Note: bolter fire deflects off anything hard if the angle is oblique enough. Contrary to Imperial propaganda you can't actually destroy any real military vehicle with one.
Kevlar's good at stopping bullets and not much else. Having bullet-resistant guts won't do a damn thing against hell's heels.
and even that is only the case if the kevlar is rinforced with steel plates (at least against rifles etc.).
On ly thing kevlar is extremly good at is at not tearing under strain. If you twist ot kink the fibres they will rip easily