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:

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.