My answer: treat it, don't conceal it. I think that concealer only draws attention to pimples - or else I'm not that skilled in applying concealer.
Paula Begoun's 2% Exfoliating BHA Lotion
When I got pimples, I used Paula Begoun's Exfoliating 2% Beta Hydroxy Acid Lotion and applied a thin layer of mild 5% or less benzoyl peroxide lotion (less expensive store brands are available) to treat the pimple. I would apply hot water compresses (soak a washcloth in steaming hot water, let cool slightly, wring out the washcloth, and press) to bring the pimple to a head. I don't recommend that others follow my example, but I have been known to pop the pimple once it comes to a head.