Thank you very much for your question. It is our pleasure to answer it.

In the name of Allah, We praise Him, seek His help and ask for His forgiveness. Whoever Allah guides none can misguide, and whoever He allows to fall astray, none can guide them aright.

Dear sister, with regard to yellowish or brownish discharge before the menses, if it comes at the usual time of the period or a little before, and it is accompanied with menstrual pain or cramps and is immediately followed by menstrual bleeding, then it is part of the menses and you should refrain from praying and fasting at that time. This is like when a brownish discharge comes for one or two days, accompanied by menstrual cramps, then the bleeding starts on the third day; all of that is menses. This is the most correct scholarly opinion concerning this issue. 

Therefore, you should stop offering prayer, fasting, touching the Mushaf or going to the Masjid and wait until you finish your monthly period. 

And Allah Knows best.