At all times, for all cards, assuming everyone can see all cards and play perfectly:
a number in red means the contract is going down that many tricks if this card is played now;
a number in green means the contract is making with that many overtricks, if this card is played now;
an equals sign [=] in green is equivalent to "green zero", i.e. the contract is making exactly, if this card is played now. [=] is probably used instead of [0] because traditionally 2H= means "2 hearts, making exactly" (so everyone knows what it means), and because [0] "looks like" a bad thing (could be misinterpreted as "making no more tricks" or "getting a zero" instead of "making with zero overtricks").
Long live the Republic-k. -- Major General J. Golding Frederick (tSCoSI)