Color is only one way to improve the usability. I think glyphicons included in bootstrap can complement the color as an indicator of the meaning of buttons, and they should be used whenever appropriate for that very reason.
SC 1.4.1 Use of Color: Color is not used as the only
visual means of conveying information, indicating an
action, prompting a response, or distinguishing a visual
element. (Level A)
Users who have color-blindness benefit when information
conveyed by color is available in other visual ways.