Is what I did correct, repeating the Saturday prayers, and must I repeat the prayers again if I doubt my purity and see a dark color on my underwear?

If the yellowish discharge is not connected to menstrual blood, it is not considered menses. You must re-perform the prayers you missed, thinking it was menstruation, but you are not required to re-perform the prayers you performed afterward. What you have seen recently is not considered menses unless you are certain it is menstrual blood, and for this, you only need to perform istinja (definition)' (washing private parts) and wudu' (ablution). However, if the yellowish discharge is connected to blood, then it is menses, and performing ghusl (definition) (ritual bath) after its cessation is obligatory.