The Colors window in Pixelmator Pro lets you access the system color palettes, and all the macOS-native color pickers such as the color wheel, sliders, and pencils.
The Colors window has four sections: the different color pickers at the top (color wheel, color sliders, color palettes, image palettes, and pencils); the active color picker in the middle; the large color swatch in the lower-left to show the currently selected color; the eyedropper for picking colors; and the row of boxes to pick or save new color swatches.
Important: Colors in the color wells update based on the tool or the layer selected, so if you select a color for one tool, the colors don't change for others (except for the painting tools). This means that if you leave the Colors window open for one tool, and select another object in the canvas, clicking colors in the already open Colors window does not change the newly selected object.