Highlights basically brighten your hair by adding colors that are 2-3 shades lighter than your natural color. Colors used for highlights are gold, amber, red and honey shades. You need to look at your hair and skin tone to determine what color to use to highlight.
Lowlights do the opposite by adding slightly darker colors that blend with your hair's tone. Lowlights will deepen your hair color and are usually deep reds, coppers and plum shades. Most of the time 2-3 colors are used throughout to add dimension and shimmer.
Lowlights make highlights look more natural if you're brunette. When brown hair starts to lighten, it goes through red shades first so using red, copper or tortoise lowlights will add dimension and depth to dark hair. Ideally you should use 3 shades for darker hair.
After my own feeble attempt at coloring and mad dash to a professional, I ended up with highlights and lowlights in my hair. My hair is naturally a dark walnut color so we mixed ash blonde, caramel and copper for highlights and lowlights. I get tons of compliments on it and it looks stunning if i do say so myself!
Blondes have more options for highlights and don't necessarily have to follow the 3 color rule. Gold and copper lowlights are best to avoid washing out your complexion or making your hair look brassy.
When working with highlight and lowlight hair color ideas, keep the season in mind. Go a little darker in the winter when skin is often paler. For the summer, try a shade or 2 lighter.
Since highlights and lowlights can get complicated, it's best to consult a professional for these hair color ideas or learn as much as you can about hair coloring before trying this on your own. This is particularly true if you're going more than 2-3 shades lighter than your natural color.