If perfect hilites and hair color were only a formula, you could simply buy a box of color and do it at home. However, colorist are trained professionals. It takes years of knowledge to formulate. It is not reading a few numbers off a sheet of paper or flipping through a sample book. Colorist look at your natural color level, desired level, remaining color, face shape, skin tone and personality type. All of these things together are key components into formulating the perfect color. Additionally, these elements must be looked at every time you see your colorist. Sometimes small adjustments need to be made and these small modifications are what keeps your color consistent.
Hilites are different from hair color both in formulation and placement. Placement means how colorist put them in your hair and the number of foils that are used. If hilites are not placed close enough to the scalp you leave with what appears to be a week of re-growth. Too many foils means you lose dimension and your hair appears to be one solid color. Hilites are very difficult to replicate and again consistency is key.
Finally, your colorist should monitor your at home products, how often you shampoo/condition and what styling tools you use on your hair. It is this process and the relationship you and your colorist share that makes perfect color.