To view recently used colors while editing a layer, navigate to the Styles tab within the layer you are editing. When you click on the "fill color" or "stroke color" buttons, as long as you've changed colors in the past, it will show the other colors you used as boxes in the bottom left corner of the pop up. If you simply click on one of the boxes that has a color you used before, it will change the feature to that color on the map.