First you need to find a species of turtle, which are okay to live together. Red ear sliders for example are loners and would suffer with 4 together in one enclosure.
Then you need to find a color (imagine it like nail polish) which does not give chemicals to the organism. This you could use to paint the band onto their upper shell. I saw others use turtle friendly color to write their names or phone number onto the shell, to get the turtle back, if it breaks from the garden enclosure.
article about the risks of coloring tortoises: paint will decrease the abability of the shell to use sun rays to produce vitamin D (very important for healthy and strong shell and bones). So I would recomment to paint as less space as possible, a small line may be okay.
method paper about tracking turtles: "Juvenile turtles were marked with either a transmitter or using dots of water-based
nail polish applied to the middle of the costal and vertebral scutes of the carapace
(Figure 4-2)"(page 12)
