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You don't have to reverse colors in color scheme.

Instead, you can simply Reverse your label list and use "ReversedRow" layout:

SwatchLegend[10, Reverse@{"a", "b", "c", "d", "e", "f", "g"}, 
 LegendLayout -> {"ReversedRow", 1}, 
 LabelStyle -> {FontSize -> 20, Bold, Black, FontFamily -> "Arial"}, 
 LegendMarkerSize -> 30]

You don't have to reverse colors in color scheme.

Instead, you can simply Reverse your label list and use "ReversedRow" layout:

SwatchLegend[10, Reverse@{"a", "b", "c", "d", "e", "f", "g"}, 
 LegendLayout -> {"ReversedRow", 1}, 
 LabelStyle -> {FontSize -> 20, Bold, Black, FontFamily -> "Arial"}, 
 LegendMarkerSize -> 30]

Similarly, for PointLegend and LineLegend:

PointLegend[10, Reverse@{"a", "b", "c", "d", "e", "f", "g"}, 
 LegendLayout -> {"ReversedRow", 1}, 
 LabelStyle -> {FontSize -> 20, Bold, Black, FontFamily -> "Arial"}, 
 LegendMarkers -> {"\[FilledCircle]", 30}]

LineLegend[10, Reverse@{"a", "b", "c", "d", "e", "f", "g"}, 
 LegendLayout -> {"ReversedRow", 1}, 
 LabelStyle -> {FontSize -> 20, Bold, Black, FontFamily -> "Arial"}, 
 LegendMarkerSize -> 50]

You don't have to touch ColorData; you can simply Reverse your label list and use "ReversedRow" layout:

SwatchLegend[10, Reverse@{"a", "b", "c", "d", "e", "f", "g"}, 
 LegendLayout -> {"ReversedRow", 1}, 
 LabelStyle -> {FontSize -> 20, Bold, Black, FontFamily -> "Arial"}, 
 LegendMarkerSize -> 30]

You don't have to reverse colors in color scheme.

Instead, you can simply Reverse your label list and use "ReversedRow" layout:

SwatchLegend[10, Reverse@{"a", "b", "c", "d", "e", "f", "g"}, 
 LegendLayout -> {"ReversedRow", 1}, 
 LabelStyle -> {FontSize -> 20, Bold, Black, FontFamily -> "Arial"}, 
 LegendMarkerSize -> 30]

You can simply Reverse your label list and use "ReversedRow" layout without having to touch the color scheme:

SwatchLegend[10, Reverse@{"a", "b", "c", "d", "e", "f", "g"}, 
 LegendLayout -> {"ReversedRow", 1}, 
 LabelStyle -> {FontSize -> 20, Bold, Black, FontFamily -> "Arial"}, 
 LegendMarkerSize -> 30]

You don't have to touch ColorData; you can simply Reverse your label list and use "ReversedRow" layout:

SwatchLegend[10, Reverse@{"a", "b", "c", "d", "e", "f", "g"}, 
 LegendLayout -> {"ReversedRow", 1}, 
 LabelStyle -> {FontSize -> 20, Bold, Black, FontFamily -> "Arial"}, 
 LegendMarkerSize -> 30]