To make a woman niddah, blood must be one of the niddah colors. Red blood cells such as you describe do not make a woman niddah, since there are not enough of them to make the urine a niddah color. (Even blood that is clearly visible in the urine does not always make a woman niddah, depending on the source. A specific halachic question should be asked in such a case.)

Please write back with any further questions.