I want to make a two tone chart background

[Frank]

I am creating a chart and want the chart background above a certain value to
be of a different color (to indicate a maximum allowable value has been
exceeded). An alternative would be to jus have the gridline & axis value of
a different color/thickness/etc

 

[Luke M]

Basically, you'll need to create some dummy series. Jon Peltier gives
excellent examples of how to create horizontal bands here:

http://peltiertech.com/Excel/ChartsHowTo/HorizBands.html

Other background fills:

http://peltiertech.com/Excel/Charts/BackgroundFill.html