Ok, I'm working on this for FOTF. In the final build there will be no health displayed in the corner of the screen, instead, as the player gets badly hurt, the screen gets more and more bloody.
I realise this video is a bit short and doesn't show the effect to it's full potential. In truth, I developed this for multiplayer (because in MP there is no shot damage screen flash), I then decided to implement it into the single player.
If you collect a heath pack, the blodd goes away again, and you can't really see from the vid, but it is semi transparent.
It's only very small, so download it and check it out.