Here is an example that can get you started. The code below assumes
you have a value in cell  A7 and a bar  or column chart  showing that
value. Put this code in the sheet by right-clicking in the sheet's
tab and choosing "view code".

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$A$7" Then
Set x = ActiveSheet.ChartObjects("Chart 1") _
.Chart.SeriesCollection(1).Interior
If Target.Value < 0.25 Then
x.ColorIndex = 46 'Orange
ElseIf Target.Value <= 0.5 Then
x.ColorIndex = 3 'Red
Else
x.ColorIndex = 4 'green
End If
End If
End Sub