Make --color and --no-color exclusive

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

main.go

@@ -952,6 +952,10 @@ func (c configImpl) Color() bool {
 		return c
 	}
 
+	if c.NoColor() {
+		return false
+	}
+
 	// We replicate the helm-diff behavior in helmfile
 	// because when when helmfile calls helm-diff, helm-diff has no access to term and therefore
 	// we can't rely on helm-diff's ability to auto-detect term for color output.
@@ -1021,6 +1025,10 @@ func action(do func(*app.App, configImpl) error) func(*cli.Context) error {
 			return err
 		}
 
+		if err := app.ValidateConfig(conf); err != nil {
+			return err
+		}
+
 		a := app.New(conf)
 
 		if err := do(a, conf); err != nil {

pkg/app/validate_config.go

@@ -0,0 +1,11 @@
+package app
+
+import "errors"
+
+func ValidateConfig(conf ApplyConfigProvider) error {
+	if conf.NoColor() && conf.Color() {
+		return errors.New("--color and --no-color cannot be specified at the same time")
+	}
+
+	return nil
+}