White is o.the only gene abreviation for egg shell color I've seen is O for blue. I'm assuming it's referring to the name of the blue pigment. which... I've closed all my tabs so I can't recall the name of ATM. and I saw PR somewhere, referring to brown. which I think also refers to the pigment color name itself: protoporphyrin IX
I didn't see one for white, but I think it would likely just be covered with a - because it is a lack of pigment.
Blue is O because it is dominate to white.