Refer to figures. 
EASA learning objective 022.12.10.01.18 states: 'Explain the indications of a TA and an RA and how an RA will generate a red area on the VSI. Some variants will also include a green area. To manoeuvre the aircraft to comply with the RA, the pilot should ‘avoid the red’ or ‘fly the green’'.
EASA learning objective 022.12.10.01.06 states: ‘Explain that an RA may or may not require any active control input and the implications of reacting instinctively without awareness of actual control inputs required to comply with the RA.’

When an intruding aircraft with a Mode A/C (with altitude encoding) or S transponder poses a collision threat a TCAS II system will generate a resoultion advisory (RA) which commands a manoeuvre in the vertical plane when necessary. TCAS II RA commands are aural ('climb...', 'descend...' etc) and visual, with required rates of climb and descent being displayed to the pilot on the EVSI or the PFD vertical speed tape.

A red arc on the VSI scale indicates climb and descent rates to be avoided during an RA.  In some cases a green arc represents the recommended vertical speed to be maintained during an RA.

Resolution advisories (RA's)are classified as follows:

Preventative - An RA that doesn't require any change in the current vertical flight path.  This could involve maintaining level flight, avoiding the red areas displayed on the PFD VSI and attitude indicator, the aural command for which is ‘Monitor Vertical Speed’.  It could also involve maintaining a rate of climb or descent, avoiding the red areas on the PFD, if the aircraft was already climbing or descending when the RA was issued, the aural command in this case is ‘Maintain Vertical Speed, Maintain’

Corrective  - An RA that requires an immediate change of vertical flightpath, in addition to red areas on the PFD there will be a green band shown on the VSI indicating the required rate of climb or descent.  Aural commands include multiple permuatations of Climb, Descend, Level Off etc.

RA commands must be followed, no ATC clearance is required, RA's override ATC instructions. As the LO says: 'Avoid the Red' or 'Fly the Green.'