Code:
set_rendering(ENTITY,kRenderFxNone,0,0,0,kRenderTransTexture,AMOUNT);
AMOUNT should be somewhere from 0-255, 0 being completely invisible and 255 being completely visible
As for your other problem, I believe the colors for decals are hard-coded in and cannot be changed. This means the only way you could get green blood decals is if they already exist somewhere in the game. Have you ever seen green blood decals?
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No longer around. Thanks your support, everyone! As always:
THIS ONES FOR YOU
3000 PTS