Retouch Roots & Highlights
A: Performing a color retouch and retouching highlights at the same time is possible, but usually only by using the foiling technique since this will allow you to isolate the hair to be highlighted from the hair that needs color retouch with the base color.
I do recommend that you separate the color you plan to use in halves and mix the colors half at a time. This way, you can be assured that the color won't have the color become inert before you have finished applying it to the entire head if it takes a little longer than anticipated.
Remember that color, once mixed with the developer has a lifespan of only 30 minutes and that it usually takes 15-20 minutes to process the color onto virgin hair, so you have to be able to get the color applied pretty quickly once you've mixed the color.
If you can't find the help or you're not comfortable with the idea of working with two colors at once, or the foiling technique, you can always do your base all-over color, then reapply highlighting in a second stage after blow-drying and you should do fine. Keep track of your hair's condition, so that you don't overtax it, though, and you may want to give your hair a day or two of "rest" in between if you feel it needs it.
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See also:
How to color your hair
The foiling technique for highlights
Can I color my hair and add highlights at the same time?