The difference between cap highlights and a foil highlight
The difference between a cap and a foil highlight is the product used, the time it takes, and the total end result.
A cap highlight allows the colorist to watch the color as it lifts in your hair.
Caps allow you to apply color through the entire length of your hair, using the same strength developer, at one application.
Cap highlights allow you to remove color one stage at a time because you can watch it as it processes.
A foil highlight allows you to target just the roots or ends of your hair.
Foil highlights allow creative coloring without the colors overlapping each other.
Foil highlights allow for color control on longer, over-stressed, or over-processed hair, by allowing the colorist to adjust the strength of the developer.
When would you want a foil instead of a cap?
Your hair colorist may recommend a foil instead of a cap if your hair is damaged or over-processed, the roots may need stronger or longer developing time.
Foil may be recommended if you have long hair, because it may not be practical or comfortable to pull it through a cap.
Creative coloring can be done with either method: a foil separates the colors, with a foil barrier. Foils verses cap is considered when coloring needs more or less oxidization.
Both services are timed and priced, based on the condition, length and desired results of the hair determined by the professional doing the service.