This is how color replacement tool works. In color replacement tool we have only for Blending modes option available - Color , Hue , Saturation and Luminosity. Currently your blending mode is color . In color mode the result color takes the luminance of the base image color and the hue and saturation of the foreground painting color. This is what it does in the picture posted above.
Paint Bucket tool is the correct tool for the type of change you want. In Paint Bucket tool , set the blending mode to Normal and then just click on the Brown color.
In my post above I have explained the logic of "Color" Blending mode.
It’s helpful to think in terms of the following colors when visualizing a blending mode’s effect:
•The base color is the original color in the image.
•The blend color is the color being applied with the painting or editing tool.
•The result color is the color resulting from the blend.
Color blending mode - Creates a result color with the luminance of the base color and the hue and saturation of the blend color. This preserves the gray levels in the image and is useful for coloring monochrome images and for tinting color images.
The color replacement tool is working as per above Color blend behaviour. Hope it helps