The liquid will change either the foliage or the flower type, but not both. If you use it on a newly-planted seed before it has foliage, you will not know what changed at all until the plant grows. If you use it on a growing plant, you might see the foliage change. Then you'll know what the mutation was. If the foliage doesn't change, then you must wait until the plant blooms to see the mutation results (and hope you remember what the original flower was before you mutated it).

It's just easiest to use it on a mature, flowering plant, where the results will be instantly obvious. Then you can self-pollinate if you want to make more of that plant.