ReplaceColor Issue

I'm having a slight issue with ReplaceColor, It's changing the colors when the effect is applied.

The image is originally pure White/Black but when the effect is used it will have a more varied version of colors as shown in the following images.

Left is the image with ReplaceColor - Right is a pure image without any effects

Left is the image with ReplaceColor - Right is the image before the effect

Does anyone have any idea how I can avoid this issue or why it is occurring?

I cant say for sure but do you have "pixel rounding set to "Yes" in the properties dialog box?

I cant say for sure but do you have "pixel rounding set to "Yes" in the properties dialog box?

That was my first thought and it was turned off but still produces the same issue unfortunately.