My skin is medium-dark and quite warm - and it changes a lot over the year - so matching foundation and concealer products used to be really tough. A lot of companies just don't cater to my skin tone. Like many people, I had a raft of products that just weren't quite right - too warm, too cool, too dark, etc.. I bought the adjuster palette in an attempt to salvage those products where I liked the formula but not the shade.
Don't think it's limited to RCMA foundations - you can use this palette with any cream product (i.e. foundation or concealer). I even adjusted liquid foundations with it - I just used a thinner to turn the adjuster shades into a liquid and then mixed it through. I fixed entire pots and bottles of products - I won't have to buy anything new for ages!
Another thing you can do is create colour correctors that are perfectly matched to your skin. I made my perfect under eye corrector by taking my concealer and mixing in some yellow and red from the adjuster palette. Whatever shade you need - orange, peach, pink, green....you can do anything with this palette. I've even used the red shade straight to colour-correct really dark circles.
Like all RCMA products, it's great value. I highly recommend this for anyone who finds it hard to find a perfect match in foundation or concealer. This will save you a bundle