\version "2.25.1"

#(define-markup-command (on-color layout props color arg) (color? markup?)
   (let* ((stencil (interpret-markup layout props arg))
          (X-ext (ly:stencil-extent stencil X))
          (Y-ext (ly:stencil-extent stencil Y)))
     (ly:stencil-add (ly:make-stencil
                      (list 'color color
                        (ly:stencil-expr (ly:round-filled-box X-ext Y-ext 0))
                        X-ext Y-ext)) stencil)))

{  c''2 ^\markup { \on-color #green "pure" } s }