Замена одного цвета на другой в картинке

Imports System.Drawing.Imaging
Imports System.Runtime.InteropServices
 
Public Class Form1
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim bmp As New Bitmap("C:\Пример.png")
        PictureBox1.BackgroundImage = New Bitmap(bmp)
        Dim pxf As PixelFormat = PixelFormat.Format32bppArgb
 
        Dim rect As Rectangle = New Rectangle(0, 0, bmp.Width, bmp.Height)
        Dim bmpData As BitmapData = bmp.LockBits(rect, ImageLockMode.ReadWrite, pxf)
 
        Dim ptr As IntPtr = bmpData.Scan0
 
        Dim numBytes As Integer = bmpData.Stride * bmp.Height
        Dim rgbValues(numBytes) As Byte
 
        Marshal.Copy(ptr, rgbValues, 0, numBytes)
        Dim Startcol As Color = Button2.BackColor
        Dim ResColor As Color = Button3.BackColor
        For counter As Integer = 0 To rgbValues.Length - 4 Step 4
            If CInt(rgbValues(counter + 2)) = Startcol.R And CInt(rgbValues(counter + 1)) = Startcol.G And CInt(rgbValues(counter + 0)) = Startcol.B Then
                rgbValues(counter + 2) = ResColor.R
                rgbValues(counter + 1) = ResColor.G
                rgbValues(counter) = ResColor.B
            End If
        Next counter
 
        Marshal.Copy(rgbValues, 0, ptr, numBytes)
        bmp.UnlockBits(bmpData)
        PictureBox2.BackgroundImage = bmp
    End Sub
 
    Private Sub Button2_Click(ByVal sender As Button, ByVal e As System.EventArgs) Handles Button2.Click, Button3.Click
        Dim CD As New ColorDialog
        If CD.ShowDialog = Windows.Forms.DialogResult.OK Then
            sender.BackColor = CD.Color
        End If
    End Sub
End Class