Right now, as of 2017, there are a limited number of Emoji Flags that you can send. Of Emojis that are flags we have the: checker, yellow pennant, and two black flags, two white flags. There is one horizontal tribar of white-black-white. Incidental flags also appear with other item Emojis like a red pennant on the circus big top tent, another red pennant on a golf course, and the grey mailbox mail-indicator flags.
Surely soon more flag colours will appear and a needed rainbow flag. Yet to come: orange, green, purple, pink, brown, and others.