There are 5 type of flags you can use (and their shortcuts):

• Empty (Ctrl+0)
• Blue (Ctrl+1)
• Green (Ctrl+2)
• Red (Ctrl+3)
• Yellow (Ctrl+4)
  
  

You can flag rows 2 ways:

• just click on the sign in the very first column and keep tapping until you see what you want (1)
• right click on any cell of the row you want to flag (2) and select option "Flag" (3) then choose the one you want to use (4)