Should one use only one color, for example green, for call to action buttons across a website?
I'm going to assume that
- you that you are not referring to ALL call to action buttons and
- that you are not using the same color for all states.
Consistency is important. If you're using a green color then be consistent.
But keep in mind there are different types of CTAs and there may very well be a need to differentiate them. (In which case keeping a consistent color standard is even more important.)
There are at least two states for CTA - one the CTA is unavailable to the user; meaning that he needs to perform an action first. This is very common in forms.
If the user needs to select "A" or "B" before the system knows what to display then the CTA should be grayed out (or otherwise inform the user that this action cannot yet be taken). Once the user has done the required action then the CTA would change into its active state. (In your example: green).
Also keep in mind that green, while often associated with go, need not be used. I often use a shade of blue. In the current project I'm working on we're using 2E8DC3 for active and E4E4E4 for inactive. In case it matters :)
Of course it will be easier for the user to quickly distinguish the CTA if they are all identical. There may be exceptions depending on the color of the background for example, as long as the CTA remains well identifiable by keeping same spacing, shape, shadow etc...