As I understand it, the blue labeled fertilizer is for the blue labeled flowers, and the purple for the purple. That is how I have used it, and it seems to work. But the white labeled flowers don't need fertilizer, so what is the white labeled fertilizer for?

The level of fertilizer is related to how rare the plant is, not the color of the label.

Sunshower, you are right that the level of fertilizer is related to how rare the plant is, but the color of the label (of either plant or fertilizer) is ALSO related to how rare the plant is.

ChaoticMoria (whose avatar is wearing a favorite dress) is right about the label colors for Tiers 2 and 3. What I call "uncommon" plants have a blue label when you tap them in your inventory, and Tier 2 fertilizer has a blue name label in the store. Tier 3 plants (I call them "rare") and fertilizer have magenta (or purple) labels.

Tier 4 fertilizer, which has a white name label in the store, is apparently for even more rare plants, which have a different label color. I'm not sure many (or any) of these have been seen. Judging by the number of spaces on the far right in the Museum's plant collection, there are probably 6 of them. I'm not sure why the name label in the store is white, but IMO it is for these ultra-rare plants.

I never considered a higher level of rarity. I wonder if nobody has seen it yet, if perhaps it will come with an update or something. Hmmm

It's called the Jalapa tree