Personally I would say the best colours to use would be a variation of the colours already on the base.
That's If you want the alien grass to star and also blend in
if you start with the base in mind then using the colours you already have is a good idea.
Something like the bottom right scheme here
If you want to keep your origional scheme.
Using a colour wheel may help.
Complementary colours look good together. So do triads of adjacent colours.
Also warm colours work together.
Cool colours work together which is best for what you have going here.
From yellow to red are warm colours the other half are cold colours
to me the brown orangey colour on the gun and claws is the main thing for me that is making this scheme look off.
It's adding a warm colour to a cold pallet
Change this to a more neutral off white/bone colour and it would help this scheme a lot.
Toning down the saturation of the colours may help, but really I think ditching the warm colour from this scheme will help the most.