Asking Maros by a non Lubavitcher Rov
I have a question of Maros and we live near a larger city that has a qualified Litvishe Rov there, is it okay to show the Shailah to him or do I need to overnight it to you?
Maros can be shown to any qualified Rov who knows Maros.
If it is not a question of color but of Psak, it should be noted that the Alter Rebbe and the Tzemach Tzedek have unique rulings, specifically in matters of Niddah.
Determining whether a particular color is acceptable or not isn’t something unique to Chabad Rabbonim, although as in every matter of Halacha there might be slight variations from one Rov to another, and some might even wish to argue their position based on evidence from our Rebbeim. Nonetheless, in general principle, there would be no issue with asking another Rov whether this color is Tahor or not.
With regards to matters of Psak, for example, the actual size where stains are problematic, or whether Kinuach-an external wipe with a problematic color causes Tumah, or other Halachic rulings, there are specific positions maintained by Chabad Rabbonim based on the rulings of our Rebbeim which other Rabbonim might not necessarily agree with.