What should I do if a light yellow stain appears on my trousers or izar, even though I perform ablution for every prayer and wash my private parts and anus? Is my prayer valid if I find this yellowishness after ablution?

The discharges that exit the anus are impure (najis), and their ruling is similar to that of urinary incontinence. Therefore, complete istibra' (purification/cleansing) is obligatory, along with tying a cloth over the area and performing ablution (wudu') after the time for each prayer has begun. As for past prayers, what you prayed after performing wudu' subsequent to the prayer time commencing is valid. The impurity on the clothes does not affect these prayers due to ignorance of the ruling. However, what you prayed before the prayer time commenced is invalid according to the majority of scholars, and you are obliged to repeat it. According to Ibn Taymiyyah, there is no make-up prayer (qada') required of you due to the excuse of ignorance. The safer opinion is that of the majority.