For the missing arm i would use a knife to rough up the wound a bit, it looks too clean a cut. For blood i would have flicked it on with a tooth brush to look like spray rather than gallons of it gushing from the wound and all down the minis front :)
If you want to paint yellow, try to prime white, not black. It makes it easier to achieve good results. It also allows you to thin down your paint, so the result will look less chunky. Try to remove the molding lines with a modelling knife.
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