A color-picker or color-selection tool is part of a GUI that allows users to select colors for use in software applications. There is a widespread belief that some color spaces or models are more “natural” than others. However, this intuitively appealing idea that a color space based on the nature of color perception (such as HSL) may be preferable to one that is driven by the nature of the technology (such as RGB) has not been confirmed by all studies. In this paper we argue that it is not the color space per se that is the most important factor underlying the usability of a color-selection tool but rather the color-mixing algorithm. An experiment was conducted to determine matching performance in a color-selection tool where the sliders interacted with the on-screen color using either a direct additive model or an indirect subtractive model to control the RGB values of the on-screen color. When observers were given a limited amount of time to use the sliders to match target colors their performance was statistically superior when they used the subtractive CMY sliders than when they used the additive RGB sliders. This is consistent with some previous work that suggests that observers possess better internal models of subtractive mixing than additive mixing and that the design of color-selection tools could exploit this. It should be noted that this work differs from some related work that has looked at the influence of color space on the usability of color-selection tools. Our hypothesis is that it is the color-mixing model that relates the slider bars to the on-screen color that is important rather than the choice of color space itself.