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"Spirit Lake" is an early multifractal terrain model. The rainbow is a physical model based on Rene Descartes' seventeenth century paper in which he ray traced an idealized raindrop, thereby explaining the angular position of the primary and secondary arcs, the excess of light inside the primary bow and outside of the secondary, as well as Alexander's band of relative darkness between them. Interestingly, it wasn't until several years later that Isaac Newton discovered dispersion, thereby explaining the colors in the rainbow.
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"Medicine Lake" is the only image I've ever made that was modeled after a photograph (the rest have arisen entirely from my interaction with the computer). The colors and contrast were borrowed from the photograph -- I would never have thought of such an unworldly combination! The photograph was of a rainbow over Medicine Lake in the Canadian Rockies. The tree in the foreground is from an early L-system model by Przemyslaw Prusinkiewicz.
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Almost all of my images began life as an illustration of some model or method for image synthesis that I was in the process of developing at the time. "Evening's Curtain" is an exception: It was conceived as a color study. (In the process of making the image, I ended up developing a Fresnel reflection model...)