How do I recolor an image? (see images)

Question

How do I achieve this kind of color replacement programmatically?

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Answer 1

You'll want to use a ColorMatrix here. The source image is grayscale, all its R, G and B values are equal. Then it is just a matter of replacing black with RGB = (0, 0, 255) for dark blue, white with RGB = (255, 255, 255) to get white. The matrix thus can look like this:

1 0 0 0 0       // not changing red
0 1 0 0 0       // not changing green
0 0 0 0 0       // B = 0
0 0 0 1 0       // not changing alpha
0 0 1 0 1       // B = 255

This sample form reproduces the right side image:

public partial class Form1 : Form {
    public Form1() {
        InitializeComponent();
    }
    private Image mImage;
    protected override void OnPaint(PaintEventArgs e) {
        if (mImage != null) e.Graphics.DrawImage(mImage, Point.Empty);
        base.OnPaint(e);
    }
    private void button1_Click(object sender, EventArgs e) {
        using (var srce = Image.FromFile(@"c:\temp\grayscale.png")) {
            if (mImage != null) mImage.Dispose();
            mImage = new Bitmap(srce.Width, srce.Height);
            float[][] coeff = {
                            new float[] { 1, 0, 0, 0, 0 },
                            new float[] { 0, 1, 0, 0, 0 },
                            new float[] { 0, 0, 0, 0, 0 },
                            new float[] { 0, 0, 0, 1, 0 },
                            new float[] { 0, 0, 1, 0, 1 }};
            ColorMatrix cm = new ColorMatrix(coeff);
            var ia = new ImageAttributes();
            ia.SetColorMatrix(new ColorMatrix(coeff));
            using (var gr = Graphics.FromImage(mImage)) {
                gr.DrawImage(srce, new Rectangle(0, 0, mImage.Width, mImage.Height),
                    0, 0, mImage.Width, mImage.Height, GraphicsUnit.Pixel, ia);
            }
        }
        this.Invalidate();
    }
}