My mom used to always make these for me when I was little.  I had forgotten about it until running across this "recipe" on a craft site!  Brings back great memories.  Wish I had a fireplace.

Holiday Color for Winter Fires

Common table salt for a yellow flame
Borax for a green flame
Rock salt or calcium chloride for an orange flame

To produce the colors, simply soak logs in a solution of water and the chosen substance. Use enough water so that the substance dissolves. Be sure the logs are thoroughly dry before burning. Soak a single log in one mixture, or soak a section of a log at a time, in a different solution for multi-colors.