Can I use ColorFunction in place of color in Swatchlegend or is there any way to include ColorFunction in Swatchlegend?

For example, the command

SwatchLegend[{Directive[{"Rainbow", Opacity[0.25]}]},
  {Text[Style["f1", FontFamily -> "Times New Roman", 18, Red]]}]

shows only gray color.

Can I use ColorFunction in place of color in SwatchLegend or is there any way to include ColorFunction in SwatchLegend?

For example, the command

SwatchLegend[{Directive[{"Rainbow", Opacity[0.25]}]},
  {Text[Style["f1", FontFamily -> "Times New Roman", 18, Red]]}]

shows only gray color.