documentclass{article}
usepackage[most]{tcolorbox}
newtcbox{FOO}{colback=red,colframe=blue}

begin{document}
FOO{Something}
FOO{Something else}
end{document}

documentclass{article}
usepackage[most]{tcolorbox}
newtcbox{FOO}{colback=red,colframe=blue,fontupper=color{green}}
begin{document}
FOO{Something}
FOO{Something else}
end{document}