How can I change the color of 'hide price' button?

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

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

hide 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 Hide Price page

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