Know Your Real Shade
It's important to know what kind of red you are. Most reds fall into one of three shades: copper (orange) red, violet red, and red red, or true red. Natural reds, or natural-looking reds, are in the copper category; very dark reds are classified as violet red; and red reds are recognizable by their vibrant, true color. These last two shades do not occur in nature.
Knowing your true shade will dictate your color-palette options. If you're a copper top, you want to make sure orangey makeup and clothing won't clash with your hair hue. The same goes for violet redheads and purple shades, and true reds and red shades — and all three should be wary of pinks.
Your Color Palette
Makeup-wise, start neutral. Staples that look great on everyone are Nars blush in Orgasm or ELF blush from its studio line of blush and contouring powder ($3!) and black mascara — we love the new Lash Blast Fusion from Cover Girl and the Anastasia Lash Lifting Mascar). Green eye makeups are great on redheads, as is purple and any color you might find in an autumnal forest. For a more daring look, metallics work with every shade.
A great way to try out different brow colors without commitment or major catastrophe is with tinted mascara or a colored brow tamer like ELF's Eyebrow Treat and Tame. Aim for one shade darker than your red hair color.
Many natural redheads have freckles (which we love!). It's up to you whether you want to go with sheer coverage or a higher pigment to cover them up. But either way, make the same choice for your whole face: Don't use a super-coverage concealer in spots and sheer powder in other places, as that can end up looking splotchy.
Going from Blonde to Red?
Of course you will want to warm up your brows (see above), but you'll find you need a little less makeup in general. And while you may need to reevaluate your lipstick and blush colors (especially the pinks and reds), try orange and copper tones in addition to greens on your eyes.
Going from Brunette to Red?
Again, unless you're going to a very dark or winey shade of red, you'll probably want to soften your eyebrow color a shade or two. You can still use the cheek and lip shades you're used to, but they will stand out more that they did on you when you were a brunette. Other than that it's the same as blondes changing to red: Just try on your blush and lipsticks and get reacquainted with how they look on you.
One Product You Can't Live Without
Sunscreen! If you're a natural redhead, you shouldn't leave the house without it.