The MakeMask subroutine uses the GetDIBits API function to load the source image into a device-independent bitmap (DIB). It processes the image's pixels and uses the SetDIBits API function to save the results into a mask PictureBox.

Private Sub MakeMask(ByVal from_picture As PictureBox, _
    ByVal mask_picture As PictureBox, ByVal from_color As _
    OLE_COLOR, ByVal to_color As OLE_COLOR, ByVal _
    other_color As OLE_COLOR)
Dim bitmap_info As BITMAPINFO
Dim pixels() As Byte
Dim bytes_per_scanLine As Integer
Dim pad_per_scanLine As Integer
Dim X As Integer
Dim Y As Integer
Dim r As Byte
Dim g As Byte
Dim b As Byte
Dim to_r As Byte
Dim to_g As Byte
Dim to_b As Byte
Dim other_r As Byte
Dim other_g As Byte
Dim other_b As Byte

    ' Make RGBTriplet values for the colors.
    UnRGB to_color, to_r, to_g, to_b
    UnRGB other_color, other_r, other_g, other_b
    UnRGB from_color, r, g, b

    ' Prepare the bitmap description.
    from_picture.ScaleMode = vbPixels
    With bitmap_info.bmiHeader
        .biSize = 40
        .biWidth = from_picture.ScaleWidth
        ' Use negative height to scan top-down.
        .biHeight = -from_picture.ScaleHeight
        .biPlanes = 1
        .biBitCount = 32
        .biCompression = BI_RGB
        bytes_per_scanLine = ((((.biWidth * .biBitCount) + _
            31) \ 32) * 4)
        pad_per_scanLine = bytes_per_scanLine - (((.biWidth _
            * .biBitCount) + 7) \ 8)
        .biSizeImage = bytes_per_scanLine * Abs(.biHeight)
    End With

    ' Load the bitmap's data.
    ReDim pixels(1 To 4, 1 To from_picture.ScaleWidth, 1 To _
        from_picture.ScaleHeight)
    GetDIBits from_picture.hdc, from_picture.Image, _
        0, from_picture.ScaleHeight, pixels(1, 1, 1), _
        bitmap_info, DIB_RGB_COLORS

    ' Modify the pixels.
    For Y = 1 To from_picture.ScaleHeight
        For X = 1 To from_picture.ScaleWidth
            ' See if we need to change this pixel.
            If (pixels(pixR, X, Y) = r) And _
               (pixels(pixG, X, Y) = g) And _
               (pixels(pixB, X, Y) = b) _
            Then
                pixels(pixR, X, Y) = to_r
                pixels(pixG, X, Y) = to_g
                pixels(pixB, X, Y) = to_b
            Else
                pixels(pixR, X, Y) = other_r
                pixels(pixG, X, Y) = other_g
                pixels(pixB, X, Y) = other_b
            End If
        Next X
    Next Y

    ' Display the result.
    SetDIBits picMask.hdc, picMask.Image, _
        0, from_picture.ScaleHeight, pixels(1, 1, 1), _
        bitmap_info, DIB_RGB_COLORS
    picMask.Picture = picMask.Image
    picMask.Visible = True
End Sub