Change fill color of radio buttons

I know this is very basic js but I am attempting to change the color of 3 groups of radio buttons based on the mouse up action on another specific radio button in a different group.  After the color change is set i am then resetting the group and setting it to read only but the trouble I am having is setting the fill color.  I am sure my syntax is incorrect but I have googled and can not find one that will work.  I am attempting to set the fill color to gray.  Any assistance would be appreciated. Thank you.

this.getField("Group2").event.target.fillColor = "150,150,150";

this.resetForm(["Group2"]);

this.resetForm(["Group3"]);

this.resetForm(["Group4"]);

this.getField("Group2").readonly = true;

this.getField("Group3").readonly = true;

this.getField("Group4").readonly = true;