I'm considering ordering a unit with highlights, my question is how does one color the rest of the unit when it needs it without coloring the highlights?

Don't think you can. When getting a unit with a bit of grey (gray) in it some companies use synthetic hair for the grey. You can then color the unit and it won't affect the grey.

Best would be to use the unit only for three or so months and then replace it with a new one, before the color fades too much.

Im wondering if you couldn't do the same using synthetic for the highlight color. May be John can address this.

I think units can get away with synthetic grey because everybody knows grey hair is naturally different in texture etc, but adding too much synthetic hair in other colours can look odd because they reflect light differently.
I wouldn't advise it.

Midwestfarmboy,
You can do hightlights in synthetic hair. But there is one caveat: the factory will not use more that 25% of synthetic hair in a unit. The reason is that synthetic hair above that percentage does not blend well with real human Remi hair. You can use synthetic is smaller amounts for gray and highlights but when it gets above 25% the difference becomes noticeable. Synthetic hair does not reflect sunlight like real human hair and when the wind musses up your hair the synthetic reacts differently and it looks fake. Small amounts are fine--25% and above is a no-no.
Regards,
John
http://toplace.com/

Thanks for the info John I was unaware of that fact.