There are 5 type of flags you can use (and their shortcuts):
Flag
Shortcut
Empty
Ctrl + 0
Blue
Ctrl + 1
Green
Ctrl + 2
Red
Ctrl + 3
Yellow
Ctrl + 4
You can flag rows in 2 ways:
Click on the Flag icon
in the very first column and keep tapping until you see the colour you want
Right click on any cell of the row you want to flag and select
Flag
option then choose the one you want to use