Side SLip or Slip-Skid
My thoughts may be a little bit controversial,
First, I consider, Airbus naming the top indexes, when yellow, "Sideslip Index" is not correct. When yellow, they are indication of lateral acceleration, in simple words they indicates the turn rate and bank angle are consistent (coordinated turn) or not. As in conventional ball, if airplane turn rate is more than the bank angle vs. speed combination required (which can be simply-trigonometrically calculated, check google), the lower trapezodial index move out of turn, which is known as "skid". if airplane turn rate is less than the bank angle vs. speed combination required, the lower trapezodial index move inside of turn, which is known as "slip". Normally this kind of coordianation is either naturally accomplished by aerodynamic design of the airplane (with a little bit help of pilot at low speeds) or by the help of yaw damper (one of the multiple functions of yaw amper is Turn ccordination) for most transport category airplane. In fact Boeing names theses indexes as Slip-Skid Indicator.
When one engine fails (assymetric thrust), without any rudder application, it is possible to fly with a constant heading. However this is possible with quite a lot bank to running engine side. In this case, not only a lot deflected ailerons and spoilers, but also a Relative Wind not hitting the nose of the airplane which is known as "Sideslip", creates a lot of drag. The ideal minimum drag with single engine is possible with slight bank on good engine side and adjusting the rudder so that RW hits exactly from nose. So how can a pilot understand Sideslip is "zero", so applied rudder is enough? He/she cannot use Slip-skid indicator, because in zero sideslip case it will deflect to good engine side like conventional ball, the reason airplane is flying straight heading (no turn) but there is a slight bank angle towards good engine side (Uncoordinated turn). The classic system is a yarn located out of Windshield center, like gliders. Since this is not practical for transport category airplane, there should be another method. A 380 has side slip vanes (similiar to AOA probes but mounted vertıcally) located just on top of radome. For 320/330? That is most likely a calculated value by computers (FCC, or FMGC etc) as differentiating the sensed static pressures at either side of airplane due to sideslip. I said "most likely" because if you search Pitot/Static boom manufactures products in Google, you will see, in their explanation they claim their products can detect the "SideSlip" on the airplane. This parameter is also an item in Alpha Call Up menu known as "AOS". That is available in AMM, however I could not see in real airplane (A330) ACMS page, I guess it requires engine failure in real plane, to be calculated and displayed.
So briefly, when Indexes are blue, that is the time we can name them as "Sideslip Index", but technically not before. When they are blue, I consider AOS calculation starts to display on PFD and when centered, even with some bank to good engine side (which is a "must" for minium drag), it represents "zero sideslip" which not necessarily minimum control deflection but optimum deflection for minimum drag.