Is it permissible to pray when a small amount of yellowish or whitish discharge, outside of the menstrual period, is present? And is it necessary to change clothes? And for how long does ablution remain valid in this case?

Discharges from the woman's uterus are pure, whereas what comes out of the urinary tract is impure. Both necessitate ablution (wudu'). It is not required to change or wash clothes because of pure discharges.

Wudu' for this condition must be performed for each prayer after its time has begun, and it remains valid until the time for that prayer expires. It is not permissible to pray two obligatory prayers with a single ablution, except in the case of combining prayers.