When applying color replace to an object, it replaces the original color with the new color the user selects. The epsilon parameter is the closeness of a pixels color to original color parameter in order to change into the new color parameter. When the epsilon value is 0, and the opacity of the color-replaced object is anything below 255, the color replace effect is nullified.
Below are square objects with the color replace effect. Their original color is red, but their new color is white. Their epsilon values are 0. Each square differs in opacity.
The square on the left has an opacity of 255, so the color changed. The middle square has an opacity of 254.999, and it didn’t change at all. The right square has an opacity of 100 for comparison.
When the epsilon value is something like 0.00001, this issue is reverted.