When the control voltage at the input of the circuit is varied from 0 V to +12 V, the LED will first light up green and then gradually, via orange and yellow, turn to red.
The two sections of the bi-color (red and green) LED are driven separately: the green one by IC1a via R7, and the red one by lC1b via R8.
Opamp IC1b has an amplification of x2, which results in the red LED lighting from input voltages of about 0.5 V. This section lights at maximum brightness when Uin>Ub /2.
Opamp IC1a is an inverting amplifier with an amplification of x2. Moreover, its non-inverting input is at a level of Ub/2. When; the input voltage <Ub/2, its output is high. When the input rises above Ub/2, the green section of the LED will gradually become less bright until it goes out completely when Uin = lib.
The supply voltage should not exceed 30 V: when it is higher than 12 V. the value of resistors R7 and R8 should be altered accordingly.