Nematode Body Structure. The phrase tube-within-a-tube is a convenient way to think of nematode body structure, and also a term used to refer to a major trend in the evolution of triploblastic metazoa (Brusca and Brusca, 1990 Invertebrates).
Neutrals and Universal Colors. Let me explain in details... In general, neutrals include black, white, gray (mix black with white and you've got gray) and different shades of brown. Nowadays a color can be called a neutral based on how effectively it can blend in with other colors. Basically, they match with all colors and color combinations - and don't create much of an emotion.
You can see a few glimpses of red, blue, and yellow in this painting. But the overall effect is of natural brown and gray colors, like those you might see in rocks or in sand, dirt, and clay. TRY IT! There are a few different ways to make neutral colors.