
Redcurrants are small, translucent, tart red berries borne in clusters.
A green gooseberry brings the acid and none of the color. Where the drink wanted acidity and red together, a tart berry syrup does both, with its sugar to account for.
Blackcurrant
A different species: dark, musky and much more aromatic, where redcurrant is sharp and clean. Swapping one for the other changes the whole character of the drink, not just the color.
Cranberry
Far sharper — cranberry juice runs pH 2.3 to 2.5 against redcurrant’s 3.2 to 3.6 — and roughly twenty-five times as tannic. It presses poorly at bar scale, so buy it already pressed.
White currant
A white currant is the same berry without the red pigment, usually a shade sweeter and softer in acid. It suits a drink meant to stay clear, and it gives up the color that is half the reason for using redcurrant at all.
A few days refrigerated, unwashed, in the container they came in, since washing early makes them collapse. They freeze well on the stem and thaw soft, which is fine for pressing and no good for garnish.