So,  here is a quick sketch. 

You will create an inline resource that will perform your webservice call and then update the CSS on a partiular class based on the outcome,  or update a variable with text based on that outcome. 

Then your button will either use the CSS Class you updated,  or will run a custom snippet to load the text associated with the varialbe you set. 

There are quite a few posts on the community about webservice calls or custom Apex inclusion etc.  Search around…