A solid layer is a single color Producer layer. A gradient layer is a Producer layer with multiple colors.
Solid Layer. You select a color for the layer. That is all. If a gradient layer is needed at a later date instead of the solid layer you are already using, you can either add a gradient layer and then re-create all of the keyframes and their settings or convert the layer to another type (see Converting Layer Type).
Gradient Layer. Generally, a gradient layer is generally more versatile than a solid color layer. You can make a gradient layer look and act like a solid layer by using a single color the start and stop color and all selections in between.
© 110414-0830, Dale Fenimore