Color is managed slightly differently for each layer type.
Each spot-color layer has a single color associated with it.
Spot-color layers appear in the Layers palette as a shaded box in the associated color: .
By default, drawing on a spot-color layer with a natural-media tool draws in this foreground color, and erasing makes a layer transparent so that whatever's below it shows through. The frame's background color will show through if no other layer is below.
This setting will stick to each tool.
The 'X' key toggles between Foreground and Background.
You can change a spot-color layer's color with the Layer > Properties command.
Full-color layers contain both color information and transparency information at each pixel. You can import a color picture into a full-color layer, then draw on it and/or mask it out - that is, make parts of it transparent. You can also make a full-color layer from a spot-color layer using Layer > Convert.
Full-color layers appear in the Layers palette as intersecting color circles: .
You can draw on full-color layers in any color. The foreground color can be set for each natural-media tool using the drop-down color menu that appears next to the Foreground button when a full-color layer is selected:
Click this button:
to see this color menu:
When you select the Background button with a natural-media tool, it simply draws in the frame background color.
The exception is the Eraser tool. By default, the Eraser is set to Background. When you draw with it on a full-color layer, the layer is made transparent. If you set the Eraser tool to Foreground, it will make the layer opaque again where you drew.
Layer > Reset Transparency makes the entire layer opaque - so it effectively undoes any erasing you've done with the Eraser tool. Layer > Matte In Transparency also makes the entire layer opaque, using a slightly different technique.
New vectors created under a spot-color layer are created with the current Foreground or Background setting. If the Foreground button is selected, the new vector will take on the color of its containing spot-color layer. If the Background button is selected, the new vector will cut out the containing spot-color layer, leaving it transparent.
Changing the color of a spot-color layer in the Layer > Properties dialog will also change the colors of all Foreground-assigned vectors underneath that layer.
When the Edit Vector tool is selected, the Foreground and Background buttons change the color assignment of the edited vector. If the vector is under a full-color layer, the Foreground color can be changed with the drop-down color menu; if the vector is under a spot-color layer, the Foreground color cannot be changed.