As a working makeup artist, one of the most trying parts is foundation color matching. I wouldn't limit the issue to women of color because to be honest whether a client is milk white or indigo, undertones will always factor in. A few makeup lines have done well to try to accommodate the diversity in skin tones artists work with, but every line wont have every shade.
I am going to be completely honest, though. Darker complexions tend to have a harder time finding the right undertone in foundations. They are either way too red, not golden enough, too warm (if the person has neutral undertones), or the shade range is too light. None of these issues should prevent an artist from working because we can mix foundation; but this has the adverse effect of making our kits pretty heavy and I know that I have caught hell trying to get a bulky kit up the subway steps of New York City. Ladies, chivalry is near extinction.
I was in Sephora a few months back and saw that Makeup Forever had foundation adjusters. They have chromatic mixes in pure red, yellow, brown, black, and white. Basically everything you'd need to make subtle changes to the undertone of a product.
I just want to let you all know that it was love at first use. In my kit I've made the switch to cream foundation but depending on the client and the coverage they want, Ill still use liquid. I currently use the Maybelline Fit Me (MFM) line and the Makeup Forever oil based drops to adjust undertone. When I say I absolutely love the results, I mean it! In the picture below I use MFM in Toffee. When I first tested the color I realized that shade wise it was the closest but the undertone was too neutral and the model had very yellow undertones. I use about two drops of the yellow chromatic mix to get that perfect undertone.
The best part about the mixers is that they do not overpower the shade of the foundation if you use a drop or two, and they work well with cream and silicone based foundations. I didn't have to worry about the product separating and the foundation ending up patchy. They also have the same line that is water based. So depending on your kit, make sure to purchase the correct product.
One thing I noticed online is that the consensus seems to be that brown sufficiently darkens a foundation. I ended up having the buy a black adjuster though because that is not the case for women of deeper skin tones. In some lines of foundations that run far too red and too light, I've used the black adjuster to darken and neutralize the color. The result is a finish that is my skin tone and undertone as I have neutral undertones.
I recommend for any working Makeup Artist that works with diverse clientele to invest in these. I purchased a red, yellow, blue, black, and white. With those 5 colors you can't go wrong and you can reduce your kit to two liquid foundation and work from there. <3
As always, makeup is fun, so Enjoy!!!