Color Change for button

No. The only way is to replace buttons with similar styled card layouts. I recommend to stay with OOTB buttons.

It is not possible to change the color of buttons via button configurations but if you want this color to be used everywhere in a particular site then you can set the Accent color of site to your likings.

In other words, maybe you can set accent color to green, and then Primary buttons will be green.  However, links will also default to green, and a lot of other things in your app.  Many of those, you can use custom colors to cover over the green, but still adds work to the whole app.

Seems an awful lot to go through to get green submit buttons.  Why do they have to be green?  How important is the green to the client?  Appian ditched it years ago.  If it's that high priority, ok.  But is it?