I'd like to ask if there's a quick and easy way to find the exact color that lies in between the two other colors. Should the AA consider both hue, saturation and brightness?
Ya it should consider all 3. I just chose a color from your existing palette which seemed about in the middle, but if I were to add a color to the palette for the AA I'd approach it by thinking about the hue, saturation and brightness seperately first. Like in the example in my previous post, you want a color between black and a color fairly close to white, so you know right off the bat that the value will be close to 50%. IN this example the hue is easy, because you can just keep it the same, but if you were AAing between 2 very different colors you can pick a hue somewhere in the middle of those 2 colors or a gray. In such a case it would probably help to lower the saturation too, because when you mix 2 colors together (atleast in paints) the resulting color is less saturated. You probably dont want your AA to be too saturated in general, because otherwise it will stand out too much instead of blend in. I wouldn't worry too much about making the AA color perfectly inbetween though. Since youre doing it manually you can make it whatever color best suites your image and palette, you just need it to be close enough to be functional.