What if you allow the primary colors to run together on the paper? Might you see some patches of secondary color; orange, green and purple? If you kept moving the brush around on the paper eventually the whole barn would turn neutral; brown and grey, but just for today let's leave those patches of primary and secondary color boldly asserting themselves. This is where the viewers come in. If they stand back a little the colors merge optically and display the entire spectrum all at once. The first layer can look like a crazy carnival comprising too many colors, but as long as your brushstrokes are vertical this experiment will not fail! Have faith. When you add those deep, rich darks the whole thing will come together, Guaranteed.