If you've been spending time at the pool, or down the Shore, or just with your self-tanner, odds are that your winter foundation is not matching your summer face.

And the biggest foundation mistake you can make is not matching your foundation to your skin tone.

You can, of course, buy a new summer foundation to match your new summer shade. I don't, though, and here's why.

It's expensive! I usually have two or three foundations in rotation at any one time. (I'm currently using and loving the Urban Decay Naked Skin Weightless Definition Liquid Makeup, $40 at Sephora, and the bareMinerals bareSkin Pure Brightening Serum Foundation Broad Spectrum SPF 20, $29.50 at Sephora.) I don't want to spend even more money to buy multiple colors.

And I'd need MULTIPLE multiple colors. As your skin color changes with the seasons, it doesn't just go from light to dark overnight. (OK, OK, it does exactly that if you self-tan.) But if your summer shade is natural, it's also gradual. So owning shades 100 and 200 wouldn't be enough — you'd need shades 125, 150 and 175 to ensure a perfect match every time.

So what to do?

Each day, I custom mix my foundation with a liquid bronzer (I use the bareMinerals bareSkin Sheer Sun Serum Bronzer, $28 at Sephora) to ensure a perfect shade match. That may sound laborious and time-consuming, but I promise it's not.

Right now, my skin is (for me) pretty dark. To get my current shade, I mix two pumps of the Naked Skin foundation and three drops of the Sheer Sun Serum on the back of my hand, then apply as usual with a flat-top kabuki brush (I use the up & up Complexion Brush, $6.99 at Target). Perfect match.

As my tan fades, I'll use a little more foundation and a little less bronzer. Should I get more color on my face, I'll use a little more liquid bronzer.

If all this mixing sounds a little too much like high school chemistry to you, don't worry: There's wiggle room. Final foundation shade not dark enough? Use a little extra bronzer. (I love the Too Faced Chocolate Soleil Matte Bronzer, $30 at Sephora.) Got a little heavy-handed with the liquid bronzer? Buff on a layer of translucent powder (try the bareMinerals Mineral Veil, $22 at Sephora) to diffuse the bronze.

You'll be able to match all the shades of your summer skin.

Alison Smith loves the sun, but reminds you to use sunscreen. Got a burning beauty question? Email asmith@philly.com.