For some reason the next option does not work in Nimbus L&F:
UIManager.getDefaults().put("RadioButton[Disabled].textForeground", radioButtonColor);
But the next commands work fine:
UIManager.getDefaults().put("RadioButton[Enabled].textForeground", radioButtonColor);
UIManager.getDefaults().put("CheckBox[Disabled].textForeground", checkboxColor);

See the commented line in this test
test/jdk/javax/swing/JRadioButton/4314194/bug4314194.java

See discussions of some related changes here:
https://github.com/openjdk/jdk/pull/10177#issuecomment-1276765550
https://github.com/openjdk/jdk/pull/10755#issuecomment-1285168651