Red knows that Yellow has the most popular flavor and tries everything he can to squash it. His angry demeanor comes from his jealousy that his coworker is far more popular. He wittily will slam him down and keep his ego down because Yellow is too stupid and humble to realize his flavour is better.
Peanut has the highest rating and vote across many polls, with one only beaten by Peanut Butter which Yellow should've been rightfully chosen as the cover character.
Red prevents Yellow from having promotions because he knows it's the superior variety. This can be evident in the recent promotional M&M blocks where one variety is missing, Peanut. Peanut would outsell the rest of them if it came in chocolate block form, including original.
It's a conspiracy held by Red to keep Yellow demotivated.