I have a plastic rubbery type mouth guard and it has started to turn yellow so is it okay to still use it.?

I have only cleaned it with cold water because that is what they told me at the dentist. Is it really okay to use a tooth brush to clean it because I am worried I might split it. The dentist never told me to use mouth wash or anything else. He just said not to clean it with hot water because that would change the shape of it. My mouth guard also has some holes in it.