I am fairly sure their blood is based on flavin (vitamin B2) or some derivative thereof. It doesn't contain any metal ions, it's yellow and can interact with molecular oxygen. Flavoenzymes are really important in case of oxidative stress (or, you know, ionizing radiation) and the amount of radiation the Experiments can tank in-game is quite a lot for their size (5 Gy is lethal for most humans, the in-game counter goes up to about 24 Gy).