Does doubt about the impurity of water resulting from a clogged drain, whose color has changed without certainty that the change is due to impurity, and without a clear smell of impurity, fall under the ruling of presumptive purity (istishab al-asl)?

If the water has changed and there is doubt whether its change was due to impurity or something pure, the original state is its purity, and it is judged as such unless there is certain conviction that its change was due to impurity. It is advisable to discard obsessive thoughts and turn away from them.