The palette of colors, as assigned, include three warm and three cool primary pigments. Those who purchased a packaged set will have many but not all of these choices. When a color is considered warm, it tends towards the red half of the spectrum. While a cool color tends towards the blue side. While this difference can be subtle, it is important to recognize this difference among your collection of tubes to match the colors of your subject matter. Blues such as Ultramarine are considered warm - their pigment is shifted towards red and they lean towards a purple hue. Blues such as Turquoise push the other way and can feel almost green. Crimson is a very blue red. Scarlet and very orange red. Lemon yellow is very cool, while Cadnium Yellow quite warm. If you have only one tube of a primary color, use it for both color wheels. NOTE: Yellow Ochre is not a primary yellow and should not be used as such.