If you are using a simple line-art image to stamp, without ink, you can fill in the impressed lines in one of two ways. In both cases you need to have done the impression, cutting and baking first. Then you can fill the dips with another colour of clay and bake again. You may need to sand down after the second bake for a good image. You can also fill in with embossing powder.
The best way to do this is to use a nice wet fine felt-tip pen, and go over all the lines you want filled with powder. Add the powder, brush it off anywhere it's not meant to be, then bake again. You can emboss just bits of the image, bake, and then do the rest in another colour.
Try this: Take two colours of blue clay, marble together to make a swirly sky. Take some brown and marble it with bronze for soil. Lie them so that the sky is above the soil, roll out to less than a quarter inch deep. Stamp a tree on it, roots in soil head in sky. Use one with fruit if you have it. Cut a circle, oval or something around the tree, so you can see your great sky and soil, and bake. When it's cool, put ink into the dips for the trunk and branches and add copper embossing powder. Bake. When cool again, do the leaves in green. If you have fruit, go round a third time with red or yellow.