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.
The primary colors of red, yellow and blue mixed together produce gray. Mixing the secondary colors of orange, violet and green together will also produce gray. Two complementary colors directly across from one another on the color wheel make gray as well. Black can be mixed with any color to neutralize it, with the result being gray with the original color's undertones. Learn more about Colors.