If you use Bootsrap with LESS, you can simply do:
.btn-primary { .buttonBackground(@yourColor, @yourColorDarker);
If not, you simply override the button class that you want to change the color of:
.btn-warning { background-color: Your color; }
Also, since it appears, you want to change the color of the button to green, why aren't you using the .btn-success class like this:
<%= button_tag "Hello", :class => "btn btn-success" %>
Source: Styling Twitter Boot Buttons
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