Stock Control - Alert user to low stock levels.

I am trying to make a stock control sheet. I need it to change the colours of corresponding cells when the stock gets to a certain level.

For instance:

• 100%-50% - High, Green Box
• 50%-25% - Medium, Yellow Box
• 25%-0% - Low, Red Box

Is there a certain formula or tool I need to use? I'm not very good with Excel but decent with maths.

I have attached the spreadsheet I'm using.