What is the algorithm to create colors for a heatmap?

Question

Assuming values are normalized from 0 to 1, what is the algoritm to get a color to create a heatmap like this?

1 is red, .5 is green, 0 is dark blue.

Working in RMagick / ImageMagick.

Answer 1

Here is a JavaScript code snippet to generate CSS hsl color code from [0, 1] value

function heatMapColorforValue(value){
  var h = (1.0 - value) * 240
  return "hsl(" + h + ", 100%, 50%)";
}

This algorithm is based on the 5 color heatmap,

In this algorithm, the colors corresponding with values are

0    : blue   (hsl(240, 100%, 50%))
0.25 : cyan   (hsl(180, 100%, 50%))
0.5  : green  (hsl(120, 100%, 50%))
0.75 : yellow (hsl(60, 100%, 50%))
1    : red    (hsl(0, 100%, 50%))

So simple!