What makes you think you can not already do this?
You can change the colour, line style and weight.

See Jon Peltier's page which explains
http://peltiertech.com/Excel/ChartsHowTo/ErrorBars.html

Cheers
Andy

hiker wrote:
I would like to be able to customize error bars in MS Excel (change their
color to match the color of the original chart).

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Andy Pope, Microsoft MVP - Excel
http://www.andypope.info