Value of GetPixel(): c#

Value of GetPixel()

In my program I have a statement to get the colour of a pixel as follow:
Color pixelColor = myBitmap.GetPixel(pixelXY.X, pixelXY.Y);

When I display value of the selected pixel using:
label1.Text = pixelColor.ToString();

I notice the value is different from the actual RGB value.
For example,
RGB value for violet is
[238, 130, 238]
but the value display in label1 is
[221, 160, 221]

May I know how to get the correct RGB value of a pixel?

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Generally speaking, the format of a bitmap can effect the rgb values of the pixel in it.

As one example, a bitmap using an alpha channel can cause the expected RGB values to be different then what is expected since the pixel color is the result of a calculation of the pixel's color and it's alpha value.

Some information about the Alpha channel ( from http://www.aurigma.com/Support/DocViewer/2/AlphaChannel.htm.aspx )
==============================================================================
Pixels of image can store information about their opacity along with color. It allows to create images with transparent or semi-transparent areas. It is very useful when you create such imagery as non-rectangle logos or watermarks which you will put on another images. There are two ways to specify transparency of a pixel - alpha channel and color key .

Color Keys
Color key is a color which is treated as transparent during displaying or blending. This technique is widely used in DirectX programming, however Graphics Mill almost does not use it (it uses alpha blending instead). It is used only when Graphics Mill saves indexed image into format which supports transparency only in color key mode (such as GIF).

The main disadvantage of this method is that it cannot save semitransparency. That's why colorkeying cannot produce smooth blend between edge of opaque and transparent area. That's why Graphics Mill use another method: alpha blending.

Alpha Channel
Instead of appointing some color to be transparent, each pixel stores information how much does it opaque. This information is called alpha channel and stored along color channels. The most wide-spreaded alpha-compliant pixel format is Format32bppArgb. Each channel (including alpha channel) occupies one byte. When alpha = 255 (max value work this pixel format), the color is treated as fully opaque, when alpha = 0, the color is fully transparent (during blending background color is not changed). For alpha values between 0 and 255, color is treated as semi-transparent and during blending each channel of the result color is calculated using the following formula:

result = (foreground*alpha + background*(max-alpha))/max,
where max = 255 for this pixel format.

            Bitmap bitmap = new Bitmap( 1,1);
            bitmap.SetPixel(0,0,Color.FromArgb(238, 130, 238));
            Color color = bitmap.GetPixel( 0, 0 );
            
            MessageBox.Show(String.Format("Colors are R({0}), G({1}), B({2})", color.R, color.G,color.B ));
 
            bitmap = new Bitmap( 1, 1, PixelFormat.Format32bppPArgb );
            // this format has 32 bits per pixel.  8 bits is used for green, 8 bits is used for red, 8 bits is used for blue
            // THE IMPORTANT PART--The red, green and blue components are premultiplied according to the  alpha each is used for 
            bitmap.SetPixel( 0, 0, Color.FromArgb(23, 255, 255, 255 ) );
            color = bitmap.GetPixel( 0, 0 );
            MessageBox.Show( String.Format( "Colors are R({0}), G({1}), B({2})", color.R, color.G, color.B ) );
 
            bitmap = new Bitmap( 1, 1, PixelFormat.Format24bppRgb );
            bitmap.SetPixel( 0, 0, Color.FromArgb( 89, 255, 255, 255 ) );
            color = bitmap.GetPixel( 0, 0 );
            MessageBox.Show( String.Format( "Colors are R({0}), G({1}), B({2})", color.R, color.G, color.B ) );

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Value of GetPixel()

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