They're alive ! After Effects 6 solids seem so smart. They name themselves and place themselves neatly into the Solids folder. Sometimes though, as cool as it is for a layer to know exactly what shade of green it is, you need a more descriptive name. Create a new solid: Layer>New>Solid or Cmd+Y (Ctrl+Y) and name the layer in the Solid Settings Dialog Box. You can call up this box again by choosing Layer>Solid Settings or Cmd+Shift+Y (Ctrl+Shift+Y).
Solids can be reused so there is no need to keep creating them. You can copy and paste solids or adjustment layers within a comp and from comp to comp.
Solids can be used like instances. Changing one can change all duplicates of the solid. This is useful if you use a colored solid with a blending mode to create a color cast in several compositions and your client wants you to change the hue.
Not creating a ton of solids and adjustment layers also keeps your project file size down.
Under AE 6.5, solids can also be continuously rasterized. This will enable the edges to hold up better when you've scaled or positioned a solid.
This is useful when using masks
Be sure to click the Collapse Transformations/Continuously Rasterize switch so that it is on.