Set 24 shot cups upright in a muffin tin or on a rimmed baking sheet so they stay steady while you pour.

Stir the lime gelatin into 1 cup boiling water until fully dissolved, about 2 minutes, with no grains visible.

Whisk in 1/2 cup cold water and 1/2 cup vodka.

Pour a thin layer, about 1/2 inch, into each cup, then refrigerate 30 to 40 minutes, until firm to the touch.

Dissolve the lemon gelatin in 1 cup boiling water, whisk in 1/2 cup cold water and 1/2 cup vodka, and let it cool to room temperature.

Once the green layer is firm, pour an equal layer of the yellow mixture over it, then refrigerate another 30 to 40 minutes, until firm.

Dissolve the grape gelatin in 1 cup boiling water, whisk in 1/2 cup cold water and 1/2 cup vodka, and cool to room temperature.

Pour the purple layer over the firm yellow layer, filling each cup almost to the rim.

Chill at least 2 hours, until all three layers are fully set and spring back gently when pressed.

Keep refrigerated until serving, and peel each cup away or let guests push the shots out themselves.