I have two questions:
First: Is it permissible to wear colored underwear, I mean dark or simply not white?
Second: My problem is that I wear my underwear today, for example, and then a day or two later I find a light yellowish spot in the front and a light brownish spot in the back of my underwear. I do my best to clean myself after relieving myself and continue washing the places of impurities for quite a long time until I become sure that they are clean. This permanently happens to me. What is the sharia ruling? Do I have to change my clothes every two days – I mean whenever I see these colors? It should be taken into consideration that I have doubts that I may be afflicted with parasites around the anus area because I feel itching and find an unpleasant odor there that is different of that of excrements. Sorry for these long details and for mentioning these words. Please help me. If these colors are judged as impurities, am I required to make up for all past prayers taking into consideration that I do not know how many days I have to count? Also, I do not even know till now whether such traces are impurities that invalidate the ablution and prayer or not.

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.