The below sentence was corrected by placing "does" in the end after one.
What is wrong without "does"? What is the exact rule or semantic I will need to keep in mind to catch this types of error.

"The blue dress looks more flattering on you than the red one." (This example is taken from Manhattan SC book)
Thanks

we are not comparing the red dress with the blue dress; we are comparing how both these dresses look on the individual. therefore "does" is required to make proper comparision.

Yep! Thanks to the guest for a nice explanation.