Refer to figures.
COLOUR CODING OF THE ASI

White arc

This arc extends from V_SO (stall full flap) to V_FE (maximum speed with flaps extended), and marks the flap operating speed range.

Green arc

This arc extends from V_SI (stall clean) to V_NO (normal operating speed), and is the normal operating speed range.

Yellow

This arc extends from V_NO to V_NE (never exceed speed), and denotes the cautionary speed range. Operations within this speed range should not be carried out except in smooth air.

Red Radial line

This line marks V_NE.

MANOEUVERING SPEED (V_A) (Figure 2)

V_A - Maximum design manoeuvring speed and is the highest speed where the aircraft will stall before it exceeds the maximum load factor

THE STALL

The C_L of an aerofoil increases with angle of attack up to a maximum (C_LMAX). Any further increase above this stalling angle, or critical angle of attack, will make it impossible for the airflow to smoothly follow the upper wing contour, and the flow will separate from the surface, causing C_L to decrease and drag to increase rapidly.

A stall is caused by airflow separation. Separation can occur when either the boundary layer has insufficient kinetic energy or the adverse pressure gradient becomes too great.

STALL RECOVERY

To recover from a stall or prevent a full stall, the angle of attack must be decreased to reduce the adverse pressure gradient. This may consist of merely releasing back pressure, or it may be necessary to smoothly move the pitch control forward, depending on the aircraft design and severity of the stall.

Allow airspeed to increase and recover lost altitude with moderate back pressure on the pitch control.

Pulling too hard could trigger a secondary stall, or worse, could exceed the limit load factor and damage the aircraft structure.

At low airspeeds the stall may occur before reaching the limit load factor. That is why, when pulling from a stall recovery, keeping the speed between V_S and V_A will allow us to not stall again due to the increased load factor.

As angle of attack reduces below the critical angle, the adverse pressure gradient will decrease, airflow will re-attach, and lift and drag will return to their normal values.