RGB Colours .... Comparing

sorry i posted in the classic forum to begin with ...

im running the following code below to replace colours .... the code works fine but i need a way to break down RGB values to calculate if the colour is roughly in the same range as the image .... how can i do this

any help would be great
many thanks
illskills

VB Code:

Dim mycc As New ColorConverter
        pgbProgress.Maximum = 256
        For iColour As Integer = 0 To 255
 
            pgbProgress.Value = iColour
            pgbProgress.Refresh()
            If Not IsNothing(arrColoursToReplace(iColour)) Then
                For i As Integer = 1 To 1000 '# loops throught mosaics
                    If IsNothing(arrCustomMosaics(i, 0, 0)) Then Exit For
                    For x As Integer = 1 To 15
                        For y As Integer = 1 To 15
                            If Not IsNothing(arrCustomMosaics(i, x, y)) Then
                                If mycc.ConvertToString(arrCustomMosaics(i, x, y).ToString) = mycc.ConvertToString(arrColoursToReplace(iColour).ToString) Then
                                    arrColoursToReplace(iColour) = arrCustomMosaics(i, 0, 0)
                                End If
                            End If
                        Next
                    Next
                Next
            End If
        Next
        For i As Integer = 1 To 255
            If Not IsNothing(arrColoursToReplace(i)) Then
                txtmytextbox.Text = txtmytextbox.Text & vbCrLf & arrColoursToReplace(i).tostring
            End If
        Next