What is the format of the color-map file?

This is green.txt. What does each entry here mean?

0.0 0.0 0.0 1.0 0.1 0.4 0.2 1.0 0.2 0.6 0.3 1.0 0.3 0.7 0.4 1.0 0.5 0.8 0.4 1.0
  0.5 0.8 0.4 1.0 0.3 0.7 0.4 1.0 0.2 0.6 0.3 1.0 0.1 0.4 0.2 1.0 0.0 0.0 0.0 1.0

A link to an article that explains this will be great too. I searched online but I couldn't find much help. I am guessing I am not using the right search terms.

The brainbrowser colormap file is a text-based color palette look up table that maps a scalar voxel or vertex intensity to a red, green, blue color. Each row contains a white-space delimited red, green and blue value (with an optional fourth alpha value for transparency that is ignored). So in the 'green' example the darkest intensities will be black (0,0,0) to a bright green (0.5,0.8,0.4 = 50% red, 80% green, 40% blue). Colors can be interpolated between items. In this example, the optional blank line provides the inverted colormap, in case the user wishes to flip the colors.

To understand the concept, you may want to explore the documentation and live demos for NiiVue colormaps which are conceptually identical (albeit they use 8-bit 0..255 instead of 0..1 for the minimum to maximum color intensity).