Double color replace problem

[qwertyasdf]

I seem to have a small problem with the color replace tool *especially in black and white pictures* The problem seems to be that when i set the color replacer's tolerance to a certain level depending on the colors to be replaced. What happens is that the replacer changes say two shades of the color at once, but the lighter shade won't change enough to escape the tolerance so it will be replaced a second time making the light color darker than it should be. When the button is held down the single shade version is shown for a moment but then the second one kicks in, I've also tried clicking instead of dragging to no avail. While this can be gotten around by doing both shades separately, would anyone have a suggestion for conveniences purpose?