Colored markers on the strings show where to place your fingers. Each finger has its own color:
Index Finger (1st)
Light cyan markers show where to place your index finger on the fretboard.
Middle Finger (2nd)
Light blue markers indicate middle finger placement positions.
Ring Finger (3rd)
Light purple markers show where your ring finger should go.
Pinky Finger (4th)
Light pink markers indicate pinky finger positions.