surfaceMeshRenderableSeries.MeshColorPalette = new SolidColorBrushPalette()
{
    Fill = Colors.DarkGreen,
};

<s3D:SciChart3DSurface x:Name="sciChart" WorldDimensions="200,100,200">

    <s3D:SciChart3DSurface.RenderableSeries>
         <s3D:SurfaceMeshRenderableSeries3D x:Name="surfaceMeshRenderableSeries"
                                         DrawMeshAs="SolidMesh"
                                         MeshPaletteMode="Textured"
                                         Opacity="0.9">
            <s3D:SurfaceMeshRenderableSeries.MeshColorPalette>
                <s3D:BrushColorPalette>
                    <s3D:BrushColorPalette.Brush>
                        <ImageBrush Source="Smiley.jpg"/>
                    </s3D:BrushColorPalette.Brush>
                </s3D:BrushColorPalette>
            </s3D:SurfaceMeshRenderableSereis.MeshColorPalette>
        </s3D:SurfaceMeshRenderableSeries3D>
     </s3D:SciChart3DSurface.RenderableSeries>
    <!-- Axis omitted for brevity -->
</s3D:SciChart3DSurface>

public class CustomColorPalette : MeshColorPalette
{   
    /// <summary>
    /// Gets the <see cref="Texture2D" /> to apply to the <see cref="SurfaceMeshRenderableSeries3D" />, given the size passed in.
    /// </summary>
    /// <param name="size">The size.</param>
    /// <returns>
    /// The <see cref="Texture2D" /> instance to map to the surface
    /// </returns>
    public override Texture2D GetTexture(Size size)
    {
        if (brush == null)
            return null;
        var writableBitmap = BitmapFactory.New((int) size.Width, (int) size.Height);
        // TODO: Fill your WriteableaBitmap
       
        return Texture2D.FromWriteableBitmap(writableBitmap);
       
        // Alternatively. You can create brushes from WPF elements like this
        // var coloredRect = new Rectangle { Width = size.Width, Height = size.Height, Fill = new RadialGradientBrush ...  };
        //
        // return Texture2D.FromWriteableBitmap(coloredRect.RenderToBitmap());
        //
        // OR like this
        //
        // return Texture2D.FromArgb(uint[] argb, int width, int height);
    }
    public void RefreshPalette()
    {
        // Call this base class method to refresh the palette and trigger a redraw
        this.OnColorPaletteChanged();
    }
}