That is a good question. I didn’t have many red ones. I was lucky that I could understand the questions that were likely to appear in an interview. So I just ignored the extra hard or extra complicated/weird ones.
Usually if one is unable to understand a solution, it could be because there is a concept missing or a gap in knowledge. Spending some time learning the concept should help.

If you are finding that you are unable to follow a lot of easy/medium question’s solutions, you might need to go back and revise/relearn the fundamental concepts again. As for the hard questions, the solutions usually become more understandable once you have solved a lot of easy/medium ones which use the same concept.

Good luck!