Yesterday's Essential Skills post covered dyeing, and there's SO MUCH to say about it. I almost included the following, but decided that a separate post was warranted. So here it is - the postscript.
One thing I've found very, very helpful over the years is making my own dye swatches. If you purchased your dyes in person at a Tandy Leather, you probably saw little leather tags near each dye to show you what it looked like in the flesh. Leather darkens over time due to exposure to sunlight, the elements, and oils (naturally, from handling, or from leather care products). The sample in Tandy or your leather store of choice may no longer be a true representative of the dye.
Different kinds of leather will also take dye differently due to their base color, composition, etc. If there's a type of leather you use frequently, it can be really helpful to get a baseline for what a particular dye might look like on it. Sealing leather can also change it's color, depending on the type of sealer you use. If you frequently take custom orders, swatches are a great way to let your customers pick the color and finish of their pieces.
I have a little bin of roughly 1 inch by 1 inch squares of leather that I've dyed with the assortment of colors I have on hand. I've also made swatches of my Cova Color leather paints. I've labeled and dated each one on the back. In the case of the dyed samples, I've also sealed them. Each square has been divided into thirds: one third unsealed, one sealed with Satin Shene, and the last with Super Shene.
Swatches can be helpful in catching changes in formula, too! Tandy seems to have adjusted the Java dye over the years, and I can tell because these are the swatches I made in 2011: