Where to buy hair color in paris

In Germany, it would be at the Drogerie (drugstore), not the Apotheke ( pharmacy). I don' know what the French equivalent of the Drogerie is, but that is probably where you would find hair color, shampoo, etc.

I've seen it at Monoprix, in the health and beauty aids section - along with shaving cream, soap, deoderant, skin care stuff....

Monoprix will have some. If you can get to a Carrefour, they'll have some. Same goes for E.Leclerc, possibly Intermarché or any other larger store. Brands and colors available will vary from place to place. Be warned: IME French formulated hair dyes come out darker than you expect. The first time I dyed my hair here I chose a product that had an equivalent color of what I used in the US and it was much darker than I expected. I also used L'Oreal in the US, but have switched to Garnier here. The formulas are NOT the same as they were in the US.