Mung bean sprouts developed a slight purple color on day 3?
The sprouts developing a purple tinge is fine, it's a sign that they were exposed to light - they will even start turning green if they recieve more light.

This is like with white asparagus: As long as the stalks remain completely covered in the soil, they stay white. If they start to break through the surface, they develop a purple tinge in a very short time. 

See the picture below:
Some stalks are still white, some slighltly purple.