public int ItemIndex { get; }

Public ReadOnly Property ItemIndex As Integer

using DevExpress.TreeMap;
using DevExpress.XtraTreeMap;
using System;
using System.Drawing;

namespace CustomColorizerSample {
    class CustomColorizer : ITreeMapColorizer {
        Palette palette = Palette.Office2016Palette;

        public Palette Palette {
            get { return palette; }
            set {
                if(palette.Equals(value)) return;
                palette = value;
                RaiseColorizerChanged();
            }
        }

        public event ColorizerChangedEventHandler ColorizerChanged;

        public Color GetItemColor(ITreeMapItem item, TreeMapItemGroupInfo group) {
            if(item.Children.Count == 0) {
                Color itemColor = Palette[group.ItemIndex % Palette.Count];
                double itemWeight = (item.Value - group.MinValue) / (group.MaxValue - group.MinValue);
                if(Double.IsNaN(itemWeight)) itemWeight = 1;

                return Color.FromArgb(
                    (int)(itemWeight * 255),
                    itemColor.R,
                    itemColor.G,
                    itemColor.B
                );
            }
            else
                return Palette[Palette.Count - 1 - (group.GroupIndex + group.GroupLevel + group.ItemIndex) % Palette.Count];

        }

        void RaiseColorizerChanged() {
            if(ColorizerChanged == null) return;
            ColorizerChanged.Invoke(this, new ColorizerChangedEventArgs());
        }
    }
}

void ConfigureTreeMapColorizer() {
    var colorizer = new CustomColorizer { Palette = Palette.Office2013Palette };
    treeMap.Colorizer = colorizer;
}

Private Sub ConfigureTreeMapColorizer()
    Dim colorizer = New CustomColorizer With {.Palette = Palette.Office2013Palette}
    treeMap.Colorizer = colorizer
End Sub

Imports DevExpress.TreeMap
Imports DevExpress.XtraTreeMap
Imports System
Imports System.Drawing

Namespace CustomColorizerSample
    Friend Class CustomColorizer
        Implements ITreeMapColorizer

        Private palette_Renamed As Palette = Palette.Office2016Palette

        Public Property Palette() As Palette
            Get
                Return palette_Renamed
            End Get
            Set(ByVal value As Palette)
                If palette_Renamed.Equals(value) Then
                    Return
                End If
                palette_Renamed = value
                RaiseColorizerChanged()
            End Set
        End Property

        Public Event ColorizerChanged As ColorizerChangedEventHandler Implements ITreeMapColorizer.ColorizerChanged

        Public Function GetItemColor(ByVal item As ITreeMapItem, ByVal group As TreeMapItemGroupInfo) As Color Implements ITreeMapColorizer.GetItemColor
            If item.Children.Count = 0 Then
                Dim itemColor As Color = Palette(group.ItemIndex Mod Palette.Count)
                Dim itemWeight As Double = (item.Value - group.MinValue) / (group.MaxValue - group.MinValue)
                If Double.IsNaN(itemWeight) Then
                    itemWeight = 1
                End If

                Return Color.FromArgb(CInt((itemWeight * 255)), itemColor.R, itemColor.G, itemColor.B)
            Else
                Return Palette(Palette.Count - 1 - (group.GroupIndex + group.GroupLevel + group.ItemIndex) Mod Palette.Count)
            End If

        End Function

        Private Sub RaiseColorizerChanged()
            If ColorizerChangedEvent Is Nothing Then
                Return
            End If
            ColorizerChangedEvent.Invoke(Me, New ColorizerChangedEventArgs())
        End Sub
    End Class
End Namespace