Tetrad refers to 4 colors and increased dynamism in the palette. This is a rectangular tetrad. Locate it on the color wheel by pointing the tips of the rectangle to at least two colors you see in your still life. Rotate it until you see a most accommodating match. You will use all four colors in your painting even though you may not identify more than two in the beginning…
We are using yellow, green, violet, and red here. Notice that these colors are two sets of complementary contrasts: red/green and yellow/violet. To mix these colors I’ll use tube colors which mix to produce neutrals. If I were to use tube colors which do not mix well, I may forgo mixing and use black and gray to tone and shade them instead.
Cadmium yellow light and Ultramarine Violet and Alizarin Crimson and Viridian green.
The outside band will contain the tube colors next to each other mixed: Yellow and green, green and violet, violet and red, red and yellow.
Now, mix diagonals. Diagonals are complements: Red and Green.
Yellow and Ultramarine violet.
After mixing diagonals, mix across. To the right see the middle yellow-green mixed with middle red-violet. Up and down.
Now, from right to left, the middle yellow-orange with middle green violet.
Now you should have 25 blobs of paint on the palette. The central 9 will contain all 4 colors in each blob but with more of the color they are closest to and less of the color they are furthest away from and about equal the colors on either corner. Here see increasing and decreasing dabs of the colors not included in the previous cross mixes.
Now, fully mixed.
Making the grid. Same orientation as the palette.
The painting with the grid hues plus white added for tints and highlights.
Now, another painting, same palette, using each grid color with white added. Adding white softens the palette, lightening all of the values of the hues.
Tone the canvas with a neutralized mid value hue.
Sketch in shapes with hues that correspond to the general colors of the subject.
Work in the background as you go. Pay attention to temperature contrast as well as value contrast.