All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
You are permitted to wear whatever you want of underwear or outwear items as long as they neither resemble women's clothes and do not contain silk or imply extravagance.
As for the colors that you see in your underwear, you are neither required to change these clothes nor to wash them as long as you are not certain that these are impurities. Since you see a yellowish spot in front of your penis and a brownish one behind your anus, they are likely urine and waste. In this case, there is a contradiction between the original state, which is purity of the penis and the anus, and the observed state, which is the color that you see as being impurity. Some scholars view that the original state is to be considered as preponderant, so they judge that these colors are not impurities. Others, however, view that the observed state is to be given preponderance, so they judge these colors as impurities. At Islamweb, we adopt the first opinion and judge that the original state of purity is not changed except based upon certainty. According to this opinion, you are not required to make up for the past prayers.
Allah knows best.