If you're tired of store-bought purple shampoo and wish to try an alternative you've concocted on your own, it turns out that making purple shampoo at home isn't very difficult. You can formulate your own bottle with any moisturizing shampoo and purple food coloring, ideally one that is of high quality. A purple semi-permanent dye can also work, but it should be noted that it may not be as easy to source as food coloring.

Now, depending on how brassy your hair is, you might have to adjust the concentration of the food coloring. The brassier your locks are, the more drops of dye the mixture needs. If unsure, you can start with two to three drops and mix it with an entire bottle of normal shampoo. You can add more drops if you deem the color not deep enough, or if your hair direly needs serious toning. 

Since it's DIY shampoo, it may take some trial and error, so your mileage may vary. You also can't expect to have the same exact results and consistency with purple shampoos you can buy from stores, but at least you have an alternative in case you want to try something new.