Foil highlights are closer to the scalp and work on most types of hair, while cap highlights work for hair that is shorter and is not past the shoulders. Cap highlights also need the hair to be thicker for optimal results.
Those wanting to use multiple colors for highlights need to have foil highlighting because cap highlights only allow for one color at a time. Cap highlighting requires a shorter amount of time, and it tends to be a little less expensive than foil highlighting. However, foil highlights hold the color better and the results can be bolder, such as a very light platinum blonde.