if you're trying to ease a client into highlights, a great way to do so is to foil in about 9 foils on the mohawk over the color you've applied during the last 15 minutes or so of the processing.  When you pull the foils and are ready to rinse, emulsify for about 3or 4 minutes and rinse completely...You'll end up with a very natural looking "sun" lightened effect, but definitely noticeable.  I do this on every full color service I do, adjusting the processing time accordingly.  I've had great luck with this....if you try it, let me know how it works for you...