cmd/clef: fix colored output on Windows (#19889)

* Fixes #19861 - coloured output

* cmd/clef: minor formatting nit

* cmd/clef: bleah, stupid github editor

cmd/clef/main.go

@@ -53,6 +53,8 @@ import (
 	"github.com/ethereum/go-ethereum/signer/fourbyte"
 	"github.com/ethereum/go-ethereum/signer/rules"
 	"github.com/ethereum/go-ethereum/signer/storage"
+	colorable "github.com/mattn/go-colorable"
+	"github.com/mattn/go-isatty"
 	"gopkg.in/urfave/cli.v1"
 )
 
@@ -392,7 +394,13 @@ func initialize(c *cli.Context) error {
 		}
 		fmt.Println()
 	}
-	log.Root().SetHandler(log.LvlFilterHandler(log.Lvl(c.Int(logLevelFlag.Name)), log.StreamHandler(logOutput, log.TerminalFormat(true))))
+	usecolor := (isatty.IsTerminal(os.Stderr.Fd()) || isatty.IsCygwinTerminal(os.Stderr.Fd())) && os.Getenv("TERM") != "dumb"
+	output := io.Writer(logOutput)
+	if usecolor {
+		output = colorable.NewColorable(logOutput)
+	}
+	log.Root().SetHandler(log.LvlFilterHandler(log.Lvl(c.Int(logLevelFlag.Name)), log.StreamHandler(output, log.TerminalFormat(usecolor))))
+
 	return nil
 }