the "grey" syntax highlighting is specific to the Xtext grammar editor. It indeed indicates that the rule is not "reachable" from the root rule. My guess is that an enhancement request will be ignored (wontfix), as you may introduce such rules for the purpose of influencing the meta model inference.
is an example. You may want Type1 and Type2 to have a common supertype without actually having a place in your language where you want those to types to be allowed in the same location.