Mosaics - A Couple of Suggestions

 I would suggest, for building mosaics, to make the outline in whatever color you wish. If you want it to look cartoon, I would recommend using a thick, black line for the outline. If you want it to look more realistic, use a different color. (See Blog post "Autumn Trees." This is a good reference for more realistic mosaics)

Autumn Trees

The first thing I did when building this was to make the outlining of the trees in red.
Then I began slowly fill in the trees with orange and yellow.
Always try to use a darker color for the outline. This makes it stand out more against a background
(When building a mosaic, always build the outline first. This provides a guideline as you fill the colorful part.)

If you look closely at the sides of the path I have 1x1 round plates to simulate stones.