Sometimes it is useful to map values to colors. For example, intensity of color could indicate population density, agricultural yield, rainfall, or other values on a map.

The MapRainbowColor method maps a value between two extremes to a rainbow color. For example, you might want 0 to be represented by red and 100 to be represented by blue. In that case, the extreme values would be 0 and 100.

' Map a value to a rainbow color.
Private Function MapRainbowColor(ByVal value As Single, _
    ByVal red_value As Single, ByVal blue_value As Single) _
    As Color
    ' Convert into a value between 0 and 1023.
    Dim int_value As Integer = Int(1023 * (value - _
        red_value) / (blue_value - red_value))

    ' Map different color bands.
    If (int_value < 256) Then
        ' Red to yellow. (255, 0, 0) to (255, 255, 0).
        Return Color.FromArgb(255, int_value, 0)
    ElseIf (int_value < 512) Then
        ' Yellow to green. (255, 255, 0) to (0, 255, 0).
        int_value -= 256
        Return Color.FromArgb(255 - int_value, 255, 0)
    ElseIf (int_value < 768) Then
        ' Green to aqua. (0, 255, 0) to (0, 255, 255).
        int_value -= 512
        Return Color.FromArgb(0, 255, int_value)
    Else
        ' Aqua to blue. (0, 255, 255) to (0, 0, 255).
        int_value -= 768
        Return Color.FromArgb(0, 255 - int_value, 255)
    End If
End Function

The form's Paint event handler uses the MapRainbowColor method to fill the form with two rainbows. The on on top draws a rainbow shading from red to blue while the one on the bottom shades from blue to red.

' Draw rainbow colors on the form.
Private Sub Form1_Paint(ByVal sender As Object, ByVal e As _
    System.Windows.Forms.PaintEventArgs) Handles Me.Paint
    Dim wid As Integer = Me.ClientSize.Width
    Dim hgt As Integer = Me.ClientSize.Height
    Dim hgt2 As Integer = Int(hgt / 2)
    For x As Integer = 0 To wid - 1
        Using the_pen As New Pen(MapRainbowColor(x, 0, wid))
            e.Graphics.DrawLine(the_pen, x, 0, x, hgt2)
        End Using
        Using the_pen As New Pen(MapRainbowColor(x, wid, 0))
            e.Graphics.DrawLine(the_pen, x, hgt2, x, hgt)
        End Using
    Next x
End Sub