具体算法请参看《vb.net 教程 5-13 图像处理之像素处理 4》

仅红色：

    '仅红色
    'http://blog.csdn.net/uruseibest
    Private Sub btnOnlyRed_Click(sender As Object,e As EventArgs) Handles btnOnlyRed.Click
        Dim destImg As New Bitmap(sourceImg.Width,sourceImg.Height)
        Dim sourceData As BitmapData = sourceImg.LockBits(New Rectangle(0,sourceImg.Width,sourceImg.Height),ImageLockMode.ReadOnly,PixelFormat.Format24bppRgb)
        Dim destData As BitmapData = destImg.LockBits(New Rectangle(0,ImageLockMode.WriteOnly,PixelFormat.Format24bppRgb)

        Dim pSource As IntPtr = sourceData.Scan0
        Dim allBytes As Integer = sourceData.Stride * sourceData.Height
        Dim rgbvalues() As Byte
        ReDim rgbvalues(allBytes - 1)
        Marshal.Copy(pSource,rgbvalues,allBytes)

        Dim pos As Integer = 0
        Dim R,G,B As Integer

        For j As Integer = 0 To sourceData.Height - 1
            For i As Integer = 0 To sourceData.Width - 1
                B = 0
                G = 0
                R = rgbvalues(pos + 2)
                rgbvalues(pos) = B
                rgbvalues(pos + 1) = G
                rgbvalues(pos + 2) = R

                pos = pos + 3
            Next
            pos = pos + sourceData.Stride - sourceData.Width * 3
        Next

        Dim pDest As IntPtr = destData.Scan0
        Marshal.Copy(rgbvalues,pDest,allBytes)

        sourceImg.UnlockBits(sourceData)
        destImg.UnlockBits(destData)

        picDest.Image = destImg
    End Sub

仅绿色：

    '仅绿色
    'http://blog.csdn.net/uruseibest
    Private Sub btnOnlyGreen_Click(sender As Object,e As EventArgs) Handles btnOnlyGreen.Click
        Dim destImg As New Bitmap(sourceImg.Width,B As Integer

        For j As Integer = 0 To sourceData.Height - 1
            For i As Integer = 0 To sourceData.Width - 1
                B = 0
                G = rgbvalues(pos + 1)
                R = 0
                rgbvalues(pos) = B
                rgbvalues(pos + 1) = G
                rgbvalues(pos + 2) = R

                pos = pos + 3
            Next
            pos = pos + sourceData.Stride - sourceData.Width * 3
        Next

        Dim pDest As IntPtr = destData.Scan0
        Marshal.Copy(rgbvalues,allBytes)

        sourceImg.UnlockBits(sourceData)
        destImg.UnlockBits(destData)

        picDest.Image = destImg
    End Sub

    '仅蓝色
    'http://blog.csdn.net/uruseibest
    Private Sub btnOnlyBlue_Click(sender As Object,e As EventArgs) Handles btnOnlyBlue.Click
        Dim destImg As New Bitmap(sourceImg.Width,B As Integer

        For j As Integer = 0 To sourceData.Height - 1
            For i As Integer = 0 To sourceData.Width - 1
                B = rgbvalues(pos)
                G = 0
                R = 0
                rgbvalues(pos) = B
                rgbvalues(pos + 1) = G
                rgbvalues(pos + 2) = R

                pos = pos + 3
            Next
            pos = pos + sourceData.Stride - sourceData.Width * 3
        Next

        Dim pDest As IntPtr = destData.Scan0
        Marshal.Copy(rgbvalues,allBytes)

        sourceImg.UnlockBits(sourceData)
        destImg.UnlockBits(destData)

        picDest.Image = destImg
    End Sub