What I'm thinking is that each shampoo/conditioner set could be themed after its respective...element? Whatever they're called. With one of the two having short hair, the other long hair.
These are pretty much taken from the bottle designs, buuuut. First one could be the main hair colour, second streaks/tips/some other sort of secondary colouration.
long list
Angelic: white/gold
Arid: light grey/bright pink
Bloodred: black/red
Cream: cream/light blue
Dawn: light pink/cream
Dusk: dark blue/light blue
Field: dullish green/brown
Galactic: dark blue/gold
Glacier: icy blue/white
Golden: gold/white
Graveyard: dull grey/dull green
Hydrus: brown/cream
Lilac: lilac/blue
Marsh: brown/light teal
Nuclear: black/bright green
Reborn: red/cream
Spectrum: white/rainbow (that is, differing in different parts of the hair)
Sun: dark orange/gold
Twilight: black/dark blue
And if other elements (for lack of a better word) were to get hair products too:
shorter list
Blacklight: black/rainbow (same way as Spectrum)
Cherry: dark red/light green or light pink
Dark Matter: black/bright purple
Glade: green/rainbow (perhaps to a lesser extent) OR dull green/light green
Nightmare: black/grey
Silver: dark grey/light grey
Steamwork: brown/gold
Storm: dark grey/light pink
Sweetheart: light pink/brown
Chibi/Common/Scribble wouldn't really fit in the same way everything else does, but perhaps they could get something else...
See, it never made sense to me why these aren't wearable as hair. And they, like quite a few other Grooming Parlor products, are just sitting around seemingly doing nothing bar being used for collections or handed in for quests. I'll probably post ideas for some other unused products at a later date, these are the first ones that sprung to mind.
(I can make previews of the sort of colours I mean if need be, though they're either taken from the bottles or the pet colour itself.)
I could see Shampoos being short hairstyles, and Conditioners being long.
The only issue I foresee is that there are a lot of them, 19 that I counted, would they all be the same hair style recolored?
They could be, or there could be a few variant hairstyles amongst them - for example Sun and Reborn having different hairstyles, it would differentiate them a bit more as they have similar colourations. Or, all of them could have more unique hairstyles, but that would take a lot longer...