Almost all Snapchat users now have the new Snapchat which was deployed throughout the United States in February 2018. The New Snapchat pushes the Bitmoji functionality but there are still many that do not have Bitmoji linked to their Snapchat account. If you have friends that do not have a Bitmoji they likely have a head with a random color. The colors we have seen are orange, purple, blue, red, green, black, grey, white, pink and many other shades of these colors.
You will likely notice these colored heads when looking at the people that have viewed your story. If you have a large number of people that follow you but you are not following them, they are going to show up as a colored head or silhouette on your views list.
At this point, we are guessing that the colors of the heads are completely random. There is no rhyme or reason why specific colors are showing up for specific users. We have also noticed that every once in awhile there will be a Bitmoji that shows up in the friends list to go along with the colored heads. We have no idea why this is the case.