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Through this post I've learned how to make a function to map a colour gradient to an interval of numbers like

redblue = 
Blend[Transpose[{{0.5, 4.1`}, {RGBColor[0.986145, 0, 0.0272526], 
  RGBColor[0, 0.0310826, 0.516304]}}], #1] & 

I tried by reversing the Transpose to create a function to do the very opposite...put in an RGBColor between the original limits and get a number. However it failed miserably.

num = Transpose[{{0.5, 4.1`}, {RGBColor[0.986145, 0, 0.0272526], 
 RGBColor[0, 0.0310826, 0.516304]}}] &

How would one create a function to map RGBColor to a number?

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inversecolorfunc[color_RGBColor, function_] := Module[
  {cd},
  cd[val_Real] := 
   ColorDistance[color, ColorData["TemperatureMap"][val]];
  ReplaceAll[
   x,
   Last@NMinimize[{cd[x], 0 <= x <= 1}, {x}]
   ]]

inversecolorfunc[RandomColor[], ColorData["TemperatureMap"]]
(* 0.0684286 *)

inversecolorfunc[RandomColor[], redblue]
(* 0.330163 *)
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  • $\begingroup$ Ah, the function works nicely, though it doesn't retreive a value from the original boundries of 0.5 to 4.1 in redblue....rather only between 0 and 1, $\endgroup$ – morbo Aug 13 '18 at 17:53
  • $\begingroup$ @morbo By default, color gradients have a single parameter that ranges from 0 to 1. You can easily modify the domain NMinimize[{cd[x], 0,5 <= x <= 4.1}, {x}] $\endgroup$ – rhermans Aug 14 '18 at 9:12

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