Hi @Guiherme ,

No need of Client scripts if you just want to change the color of your UI Action button on the form...no client script in necessary. There's a field called "Form style" as seen in your screenshot and contains a few styles already OOB. One is "Destructive" which is red...so set your reject button to that style and that one is done.

For the Green version...you'd have to create that...please see this thread and specifically look for Chuck's response that gives details on how to add other colors: https://community.servicenow.com/community?id=community_question&sys_id=d2494725db5cdbc01dcaf3231f96...

Example :

UI Action for approve:

Mark my answer correct & Helpful, if Applicable.

Thanks,

Sandeep