Question

How can I replace a color for some parts of an image without affecting its texture?

You can see good example of the result in this screenshot

Source: https://www.leadtools.com/sdk/image-processing/functions/function.asp?id=158

Answer 1

Found the way to do it. This requires RGB<->HSL conversions (I used this class by Rich Newman for HSLColor:

using System;
using System.Collections.Generic;
using System.Text;
using System.Drawing;

namespace ColorDemo
{
   public class HSLColor
   {
       // Private data members below are on scale 0-1
       // They are scaled for use externally based on scale
       private double hue = 1.0;
       private double saturation = 1.0;
       private double luminosity = 1.0;

       private const double scale = 240.0;

       public double Hue
       {
           get { return hue * scale; }
           set { hue = CheckRange(value / scale); }
       }
       public double Saturation
       {
           get { return saturation * scale; }
           set { saturation = CheckRange(value / scale); }
       }
       public double Luminosity
       {
           get { return luminosity * scale; }
           set { luminosity = CheckRange(value / scale); }
       }

       private double CheckRange(double value)
       {
           if (value < 0.0)
               value = 0.0;
           else if (value > 1.0)
               value = 1.0;
           return value;
       }

       public override string ToString()
       {
           return String.Format("H: {0:#0.##} S: {1:#0.##} L: {2:#0.##}", Hue, Saturation, Luminosity);
       }

       public string ToRGBString()
       {
           Color color = (Color)this;
           return String.Format("R: {0:#0.##} G: {1:#0.##} B: {2:#0.##}", color.R, color.G, color.B);
       }

       #region Casts to/from System.Drawing.Color
       public static implicit operator Color(HSLColor hslColor)
       {
           double r = 0, g = 0, b = 0;
           if (hslColor.luminosity != 0)
           {
               if (hslColor.saturation == 0)
                   r = g = b = hslColor.luminosity;
               else
               {
                   double temp2 = GetTemp2(hslColor);
                   double temp1 = 2.0 * hslColor.luminosity - temp2;

                   r = GetColorComponent(temp1, temp2, hslColor.hue + 1.0 / 3.0);
                   g = GetColorComponent(temp1, temp2, hslColor.hue);
                   b = GetColorComponent(temp1, temp2, hslColor.hue - 1.0 / 3.0);
               }
           }
           return Color.FromArgb((int)(255 * r), (int)(255 * g), (int)(255 * b));
       }

       private static double GetColorComponent(double temp1, double temp2, double temp3)
       {
           temp3 = MoveIntoRange(temp3);
           if (temp3 < 1.0 / 6.0)
               return temp1 + (temp2 - temp1) * 6.0 * temp3;
           else if (temp3 < 0.5)
               return temp2;
           else if (temp3 < 2.0 / 3.0)
               return temp1 + ((temp2 - temp1) * ((2.0 / 3.0) - temp3) * 6.0);
           else
               return temp1;
       }
       private static double MoveIntoRange(double temp3)
       {
           if (temp3 < 0.0)
               temp3 += 1.0;
           else if (temp3 > 1.0)
               temp3 -= 1.0;
           return temp3;
       }
       private static double GetTemp2(HSLColor hslColor)
       {
           double temp2;
           if (hslColor.luminosity < 0.5)  //<=??
               temp2 = hslColor.luminosity * (1.0 + hslColor.saturation);
           else
               temp2 = hslColor.luminosity + hslColor.saturation - (hslColor.luminosity * hslColor.saturation);
           return temp2;
       }

       public static implicit operator HSLColor(Color color)
       {
           HSLColor hslColor = new HSLColor();
           hslColor.hue = color.GetHue() / 360.0; // we store hue as 0-1 as opposed to 0-360 
           hslColor.luminosity = color.GetBrightness();
           hslColor.saturation = color.GetSaturation();
           return hslColor;
       }
       #endregion

       public void SetRGB(int red, int green, int blue)
       {
           HSLColor hslColor = (HSLColor)Color.FromArgb(red, green, blue);
           this.hue = hslColor.hue;
           this.saturation = hslColor.saturation;
           this.luminosity = hslColor.luminosity;
       }

       public HSLColor() { }
       public HSLColor(Color color)
       {
           SetRGB(color.R, color.G, color.B);
       }
       public HSLColor(int red, int green, int blue)
       {
           SetRGB(red, green, blue);
       }
       public HSLColor(double hue, double saturation, double luminosity)
       {
           this.Hue = hue;
           this.Saturation = saturation;
           this.Luminosity = luminosity;
       }

   }
}

1. Get a reference value (in hsl) representing the color you want to replace
2. Get the hsl value for your target color
3. Get image pixels and for each pixel:
4. calculate the hsl value of the pixel, and replace it with (pixelHsl / refHsl) * targetHsl