What does the circle with colour beside the player in the formation screen mean?

as of above. what is the small colour circle beside each player in the formation screen mean?

Shows if they are comfortable in that position.

Green = strong

Orange = competent

Amber = ok ish

Red = awful

Click on a player then click positions on the left, it explains all.

Quite like that feature actually, although I wonder what happens if you train a player into a new position until he reaches his maximum capacity in that new position. Does anyone know ?

Bright Green = Natural

Dark Green = Accomplished

Orange = Competent

Yellow = Unconvining

Red = Not suitable

Just reflects what is shown on the player's position screen. The maximum level a player can reach on a new position(other than his natural) by training is accomplished. These levels just tell how good a player will play in that position. Natural>Accomplished>Competent for that position.