Reference for Niddah 39:9

כגון אנא דלא ידענא בין האי להאי לא מבעי לי למחזי דמא

is a shade like this'. When the second one was taken off from him, he said to them, 'This has a different shade'. 'One like myself', observed R. Ashi, 'Who does not know the difference between the one and the other must not act as an examiner of blood'.

Jastrow