adobe photoshop - RGB Vs CMYK : Deciphering gradient transitions in different colour modes

This issue may be related to Photoshop or to colour modes in general but this is what I am trying to figure out.

• The image below is from a CMYK file with two layers.


• It has two duo colour gradients.


• 
  

  Both gradients start and end with exactly the same hex value colours.

  
  
  
  
  1. 
     

     The top gradient is one generated from in the open file using the gradient swatch shown in the image.

     
     
  
  
  
  2. 
     

     The bottom gradient is copied from an RGB file and pasted into the CMYK file displayed in the image.

     
     
  
  
  
  

PROBLEM

The CMYK gradient clearly does not match the expected output indicted on the colour swatch.

QUESTION

1. 
   

   Why does the CMYK gradient appear to have an orange transition and not match either the output expected or that of the RGB gradient?

   
   


2. 
   

   How can the true gradient be created in photoshop from within a CMYK file?

   
   

NOTES

Its interesting to note that this does not appear to be a colour palette issue as the same thing occurs when a black to white alpha channel is created on top of a pure red layer.

The hex values used for this gradient are: #de1f26 to #ffffff

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RECREATE THIS ISSUE

These steps create the issue.

1. Create new file in CMYK mode.


2. Draw a gradient on on the left side of the image. (gradient appears orange in middle as top gradient in image above.)


3. Convert to RGB mode.


4. Draw the same gradient on the right of the image. (There is a clear difference in the gradient transition.)

Answer

This was confusing at first but the striving for information has led me to a clearer understanding.

RGB vs CMYK

There is clear discrepency between gradients in RGB and CMYK this becomes clearer when you realise the palettes used by each colour mode are drasitcally different.

Colour consists of HUE, SATURATION and BRIGHNESS

RGB

RGB uses a single HUE pallete that transitions through BRIGHTNESS (y-axis in this image) and SATURATION (x-axis in this image)

CMYK

CMYK uses a single BRIGHTNESS palette that transitions through HUE and SATURATION

RESULT

As a result of the transition through the different palettes indicated by the blue arrows create a completely different gradient even though it is still a linear one for the palette that is in use.