Powershell Excel Chart - change series colors

I have a Powershell script that reads values from an Excel sheet and then creates a chart based on the values. The script works but I want to change the colors of the series in the bar chart. In VB (using an Excel macro) the code is as follows:

  ActiveChart.SeriesCollection(7).Select
    With Selection.Format.Fill
        .Visible = msoTrue
        .ForeColor.RGB = RGB(17, 17, 17)
        .Solid
    End With

Based on what I read Powershell doesn't process RGB colors so I tried using one of the default colors:

$wb.ActiveChart.SeriesCollection(5).Format.Fill.ForeColor = "black"

Whatever variation on this I try seems to make no difference to the color of the bar chart series values. Is it even possible to change the colors with Powershell?