How to set new Color every time draw() loop is executed?

[Krish]

Hello! Freinds,
I want to set different color whenever draw() loop is executed. Am using this newly created color to set color for the curves. I am trying to increment, decrement the rgb values
say by 1 or 2 but not that much noticeable change Also they shouldn't cross their limit i.e. 0-255.

draw()
{ // r = 10; g = 255; & b = 180; are static & set previously
  color.setRgb(r,g,b);
  ..
  ... // using the color
  ....     
  r += 2;
  g -= 5;
  b -= 8;
}

draw()
{ // r = 10; g = 255; & b = 180; are static & set previously
  color.setRgb(r,g,b);
  ..
  ... // using the color
  ....     
  r += 2;
  g -= 5;
  b -= 8;
}

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Can anyone please give me a hint how to solve this.
Thanks in advance.