Believe you me, makeup won't cover up that much. Their photos are *ALWAYS* airbrushed, the lighting is rigged to compliment their complexion, and during live shows no one sees them close enough to make out any major flaws.
But goodness, look at the acne on that second photo! >_> Ironically, she probably looks that bad thanks to the 10 layers of makeup she cakes on. Can't be good for the complexion, no siree. I wear very *light* water-based makeup daily, but only around my eyes thanks to having inherited my mother's mediterranean undereye circles (I generally look like I never get enough sleep no matter how early I go to bed).
The secret to putting on makeup is to go VERY LIGHT, don't use your hands to rub it in (skin oil causes pimples more than anything), go easy on the powder, and don't put it on your forehead or cheeks. Also, bathe daily, wash your face very well with hot water to open up your pores, and use astringent or acne medication daily (depending on how oily your skin is).
Also, skip the rouge. It looks so trashy. Save the eyeshadow for special occasions -- never use unnatural colors. Light lipgloss looks way better than lipstick. Take it easy on the eyeliner, one thin line ought to do it.... and under NO CIRCUMSTANCES should you EVER use mascara. It's ugly, clumpy, smears all over the place, and is rather pointless.
And that's Kat's beauty tips for the day. Followed correctly, you can avoid the Britney and Christina looks, and achieve possible classiness.