my hands turn blue but can wash off?

for the past few years, the nail on my thumbs would turn blue. I found that if I put water on it and rub it it would go away. But recently, my hole right hand would turn blue. I can still just wash it off, but I have no idea what caused it. I don't touch anything that could do this, nor is it because of the cold. I also don't know when it will happen again. Can someone please explain to me what's going on? It scares me.