Sorry "old" thread but whilst it suggests the thread was solved, to me it clearly wasn't.

To make negative numbers in an Excel chart appear in a different colour, you can change the negative number style for the cells in the chart:
1. Select the cells you want to format
2. In the Category box, click Number or Currency
3. Under Negative numbers, select an option for negative numbers

You can also change the colour of a chart by using the Chart Styles button:
1. Click the chart you want to change
2. In the upper-right corner, click Chart Styles
3. Click Colour and pick the colour scheme you want