If this shall serve a scene effect, I feel easier to have a lamp that gives the colour change, and switch it on after a delay, possibly with a slow intensity increase. The lamp itself could have the right colour, or it could be ultraviolet and the liquid be fluorescent.
Alternately, invisible light could produce a reaction that changes a colour.
Only from mixing chemicals, the effect won't be perfect. For instance the dissolution of a powder takes time, as is known from sugar in coffee. For instance phenolphthalein would be colourless in an acidic solution, you could add and mix a basic powder, and as the powder dissolves it makes the solution basic and the phenolphthalein pink. An effervescent powder would dissolve without much mixing. But the solution will turn pink at some places sooner, for instance at the bottom if the powder sinks. A fine powder can sink ("sediment") very slowly in a viscous liquid, hence stay uniformly dispersed. The initial acidity tunes the delay. Other combinations of compounds may give nicer colours: potassium ferro/ferricyanide, etc.
A clearer onset: use chemical that change colour quickly but in a geometrical arrangement that delays their mixing. Have the colour-producing compound at the bottom of a pot, topped by some compound that disappears slowly in the solution. Or the colour-producing compound could take time to percolate through a powder.