Replacing Colour Of An Image

Solution 1:

Another solution using the ColorMatrix class.

You can use the Graphics.DrawImage overload that accepts an ImageAttributes argument.
ImageAttributes.SetColorMatrix() sets the color-adjustment matrix, optionally targeting a specific category (Bitmap, Pen, Brush etc.) and can be instructed to skip the Gray Colors, modify the Gray colors only or all Colors.

The ImageAttributes.SetThreshold() method allows to regulate the Colors cutoff point (threshold) to fine tune the Brightness.
It accepts values from 0 to 1.
When set to 0, an image is all white, all black when set to 1 (see the Docs about it).

Also consider that the "Inversion" depends on the original bitmap color pattern, so try different approaches. Sometimes, inverting the brightness can give you a better result, sometime it doesn't.

You OCR must be "trained", to verify what values suits it better.

Take a look at these articles:
Recoloring (MSDN)
ASCII Art Generator (CodeProject)

Brightness Matrix:
R=Red G=Green B=Blue A=Alpha Channel W=White (Brightness)

Modify the Brightness component to obtain an "Inversion"

    R  G  B  A  W
R  [1  0  0  0  0]
G  [0  1  0  0  0]
B  [0  0  1  0  0]
A  [0  0  0  1  0]
W  [b  b  b  0  1]    <= Brightness

using System.Drawing;
using System.Drawing.Imaging;

// ...

Image colorImage = Clipboard.GetImage();
// Default values, no inversion, no threshold adjustment
var bmpBlackWhite = BitmapToBlackAndWhite(colorImage);
// Inverted, use threshold adjustment set to .75f
var bmpBlackWhite = BitmapToBlackAndWhite(colorImage, true, true, .75f);

// ...

private Bitmap BitmapToBlackAndWhite(Image image, bool invert = false, bool useThreshold = false, float threshold = .5f)
{
    var mxBlackWhiteInverted = new float[][]
    {
        new float[] { -1, -1, -1,  0,  0},
        new float[] { -1, -1, -1,  0,  0},
        new float[] { -1, -1, -1,  0,  0},
        new float[] {  0,  0,  0,  1,  0},
        new float[] {  1,  1,  1,  0,  1}
    };

    var mxBlackWhite = new float[][]
    {
        new float[] { 1,  1,  1,  0,  0},
        new float[] { 1,  1,  1,  0,  0},
        new float[] { 1,  1,  1,  0,  0},
        new float[] { 0,  0,  0,  1,  0},
        new float[] {-1, -1, -1,  0,  1}
    };

    var bitmap = new Bitmap(image.Width, image.Height);
    using (var g = Graphics.FromImage(bitmap))
    using (var attributes = new ImageAttributes()) {

        attributes.SetColorMatrix(new ColorMatrix(invert ? mxBlackWhiteInverted : mxBlackWhite));
        // Adjust the threshold as needed
        if (useThreshold) attributes.SetThreshold(threshold);
        var rect = new Rectangle(Point.Empty, image.Size);
        g.DrawImage(image, rect, 0, 0, image.Width, image.Height, GraphicsUnit.Pixel, attributes);
        return bitmap;
    }
}