VB.Net - Lockbits - Greater Than / Less Than functions - vb.net

Question

The following code works well for what I am doing. However, it is taking longer than I need it to. The problem is that it iterates over each greater than/less than function and that takes time. I've done some research but can't figure out how to slim it all down so that it runs faster.
The pixel RGB values have to run through the following tests-
(1) If one value is greater than 250, the others have to be less than 5
(2) If one value is less than 5, the others have to be greater than 250
(3) If one value equals zero, the others have to be greater than 0
(4) The difference between any 2 values have to be less than 15 (or any other threshold I set)
(5) See if two values equal zero
Also, will doing 'Exit For' after each of these functions help?
' Create new bitmap from filepath in TextBox1
Dim bmp As New Bitmap(TextBox1.Text)
' Lock the bitmap's pixels
Dim rect As New Rectangle(0, 0, bmp.Width, bmp.Height)
Dim bmpData As System.Drawing.Imaging.BitmapData = bmp.LockBits(rect, _
Drawing.Imaging.ImageLockMode.ReadWrite, _
Imaging.PixelFormat.Format24bppRgb)
' Get the address of the first line
Dim ptr As IntPtr = bmpData.Scan0
' Declare an array to hold the bytes of the bitmap
Dim bytes As Integer = Math.Abs(bmpData.Stride) * bmp.Height
Dim rgbValues(bytes - 1) As Byte
System.Runtime.InteropServices.Marshal.Copy(ptr, rgbValues, 0, bytes)
' Retrieve RGB values
Dim RedValue As Int32
Dim GreenValue As Int32
Dim BlueValue As Int32
Dim l As Integer = 0
For x = 0 To bmp.Width
For y = 0 To bmp.Height - 1
l = ((bmp.Width * 3 * y) + (x * 3))
RedValue = rgbValues(l)
GreenValue = rgbValues(l + 1)
BlueValue = rgbValues(l + 2)
If RedValue < 5 AndAlso GreenValue < 5 AndAlso BlueValue > 250 Then
ElseIf RedValue < 5 AndAlso GreenValue > 250 AndAlso BlueValue < 5 Then
ElseIf RedValue > 250 AndAlso GreenValue < 5 AndAlso BlueValue < 5 Then
ElseIf RedValue > 250 AndAlso GreenValue > 250 AndAlso BlueValue < 5 Then
ElseIf RedValue > 250 AndAlso GreenValue < 5 AndAlso BlueValue > 250 Then
ElseIf RedValue < 5 AndAlso GreenValue > 250 AndAlso BlueValue > 250 Then
ElseIf RedValue > 0 AndAlso GreenValue > 0 AndAlso BlueValue.Equals(0) Then
ElseIf RedValue > 0 AndAlso GreenValue.Equals(0) AndAlso BlueValue > 0 Then
ElseIf RedValue.Equals(0) AndAlso GreenValue > 0 AndAlso BlueValue > 0 Then
ElseIf (RedValue - GreenValue) < 15 AndAlso (RedValue - BlueValue) < 15 AndAlso _
(GreenValue - RedValue) < 15 AndAlso (GreenValue - BlueValue) < 15 AndAlso _
(BlueValue - RedValue) < 15 AndAlso (BlueValue - GreenValue) < 15 Then
ElseIf RedValue.Equals(GreenValue) Then
ElseIf RedValue.Equals(BlueValue) Then
ElseIf GreenValue.Equals(BlueValue) Then
ElseIf RedValue.Equals(BlueValue) AndAlso RedValue.Equals(GreenValue) _
AndAlso BlueValue.Equals(GreenValue) Then
Else
MsgBox("Image is color.")
Exit Sub
End If
Next
Next
MsgBox("Image is grayscale.")
' Unlock the bitmap
bmp.UnlockBits(bmpData)

Answer 1

Grayscale colors have always this specification ( R=G=B ).
eg. #cccccc = [R:204 G:204 B:204]
If Not R = G And G = B Then
MsgBox("colored")
Exit Sub
End If