How can I change the color of 'call for price' button?

Please go to your Magento root folder and follow the path:
/skin/frontend/base/default/css/callforprice.css 

By modifying this file you'll be able to customize your 'call for price' button howsoever you need to. Here is how the file looks like from the inside:

modification of call for price button

As you can see there is not only color available for modification, but also other parameters which will help your button to organically fit the style of your web-site.

See more details on Call For Price page

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