Half of spark plug are bright red on insulators

Today I decided to change the spark plugs on my truck and I this is what I had when I was finished. All four on driver side bright red all the way around insulator and red on the electrode, and on passenger side: 1 fouled, 2 very black on insulator, and 1 with a red like the driver side but just halve of the plug. I have used no additives in the gas and I have been running unleaded and occasionally mid-grade and have never run into this issue before, except once when all of my passenger side fouled at the same time one day leaving school. Any advice would be much appreciated.