public abstract class SeriesColorizerBase :
    ChartElement

Public MustInherit Class SeriesColorizerBase
    Inherits ChartElement

class CustomSeriesColorizer : SeriesColorizer {
    List<object> metColors = new List<object>();

    public override Color GetSeriesColor(object seriesKey, Palette palette) {
        int keyIndex;
        if (metColors.Contains(seriesKey)) {
            keyIndex = metColors.IndexOf(seriesKey);
        } else {
            keyIndex = metColors.Count;
            metColors.Add(seriesKey);
        }
        return palette[keyIndex % palette.Count].Color;
    }

    protected override ChartElement CreateObjectForClone() {
        return new CustomSeriesColorizer();
    }
}

Class CustomSeriesColorizer Inherits SeriesColorizer
    Private metColors As List(Of Object) = New List(Of Object)

    Public Overrides Function GetSeriesColor(ByVal seriesKey As Object, ByVal palette As Palette) As Color
        Dim keyIndex As Integer
        If Me.metColors.Contains(seriesKey) Then
            keyIndex = Me.metColors.IndexOf(seriesKey)
        Else
            keyIndex = Me.metColors.Count
            Me.metColors.Add(seriesKey)
        End If
        Return palette((keyIndex Mod palette.Count)).Color
    End Function

    Protected Overrides Function CreateObjectForClone() As ChartElement
        Return New CustomSeriesColorizer
    End Function
End Class