Easiest way to explain it is that the color or effect is coming from 2 sources. The Green for example, just has the green basecoat for color, then a layer of clearcoat over it for gloss/protection (single coat). Same for Silver, Brown, Black, etc.
Most Pearls have an initial layer of basecoat, then a second basecoat layer that has the pearl effect (this middle layer pearl layer can also be done with clearcoat depending on the company), then the clearcoat. The Sig Red has the basecoat, then a layer of clearcoat that has been colored red (for additional color), followed the "clear" clearcoat. Both of these are what Tesla is calling multi-coat.
So the multi coat can refer to either an extra step in the basecoat or clearcoat process that provides the final color or effect on the car.