Poor Buddy! Colds are the worst because they are viral, so you really just have to wait them out- my Evie had an awful URI when she was about the same age, she hid under the sofa for a week straight.
However, the colored snot may indicate that he has a secondary bacterial infection going on. I would take him to a vet to see if he needs some antibiotics to clear his nose up if it isn't better by the end of the weekend.
Is he eating? Drinking? Hiding? Playing?
I hope Buddy heals quickly and bounces back to his old self!
Has he been to the vet? That would be my first phone call. Make sure he continues to eat and use the litter box. If he becomes lethargic or stops eating, get to the emergency vet right away.
Hi. Echoing everyone above! But, when you do take Buddy to the vet ask about the viral aspect and if they might recommend something you could give him to help reduce the chances of future URIs. Here is a link to an article discussing URIs. Upper Respiratory Infection in Cats