Fix "null" display in colored JSON output

"null" was previously rendered in "bright black", an ANSI color that is
not guaranteed to be visible at all depending on the terminal. Switch
the color to cyan to ensure that "null" is visible.

pkg/jsoncolor/jsoncolor.go

@@ -10,7 +10,7 @@ import (
 const (
 	colorDelim  = "1;38" // bright white
 	colorKey    = "1;34" // bright blue
-	colorNull   = "1;30" // gray
+	colorNull   = "36"   // cyan
 	colorString = "32"   // green
 	colorBool   = "33"   // yellow
 )