Trying to change a color of black and white video, please help!

Hi! I'm working on a black and white video and I want to color a single tear red to make it look like blood. I've tried using effects like 'Change Color' and 'Change to Color', but they don't seem to work with black and white footage. I'm still a beginner, so any tips (step-by-step would be amazing!) on how to pull this off would really help. Thanks in advance!