Today my dog was playing outside we took our eyes off him for 5 minutes.. my boyfriend was calling him and he wouldn't come, when he got closer to him he started listening. Well we realized his mouth was red, bright red! Then it just now started turning blue& purplish. He's now using his paw to sort of itch it, he's kind of just rubbing it. I don't know what it could be.. we did walk over to where he was to see if we saw anything but there was only like a fruit looking thing. We didn't pick it up. If anyone knows or has any suggestions please let me know!

If he is acting differently at all I would be going to the vet.

It may just be an irritation from whatever it was in his mouth, it could be that he ingested something poisonous or it could be a bug bite.
A few months back one of mine was bitten on the face by a bull ant, his whole face swelled up, it was really frightening, but it caused no lasting damage and the vet gave him something to ease the discomfort.

I can only suggest to take him to the vet as Shazzy suggested. Without knowing what area you are living in, we wouldn't know what fruits or plants are indigenous to your area. Some plants can be toxic to animals. So it's best to have him checked by the vet.