Show color before change

[circular17]

When we click or drag through the color wheel, looking for the right color, we get a square showing the color we're getting. Most painting programs include another square right next to it (the closer the better), showing the color before we started changing it. This feature is so popular for a reason: it's a huge help for matching/comparing subjective values when changing hues. I bet it comes from traditional painting, where we hold the brush with the newly mixed color right above to the previous one, on the palette (or above any color we want to compare it to), so we can examine the before and after visually without any mental effort. I can't stress enough how much that helps.

From: https://forum.lazarus.freepascal.org/index.php/topic,69731.msg542794.html#msg542794