Before 8th Edition brought in Keywords and locked Sub-Faction Characters, Relics, Traits etc, the colour of your army had no bearing on the rules you used. The rules you acquired came from the Characters you included (think the 5th Edition Space Marine codex).
Now, it seems GW is trying to make certain colour schemes matter. This is exceedingly clear with the upcoming Space marine codex but can be retroactively applied to other codexes of 8th edition.
My example is thus: in the Dark Angels codex it states:
- "If a unit is drawn from a Dark Angels Successor Chapter, simply substitute the 'Dark Angels' keyword on every datasheet with the name of your Dark Angels successor.
- Note you cannot do this for named Characters. Azrael is the Chapter Master of the Dark Angels - not any Successor Chapters
- Successor Chapters may only give one Character a Heavenfall Blade
It seems that a similar thing is being brought in with the new base Codex in that if you don't clearly have Ultramarines but want access to everything save for named characters you can select a trait that means your Chapter is Ultramarines in all but name. With older codexes with the classic 5-6 sub-faction traits it says you can simply choose the trait that matches your army with the implication that if you play a faction with rules you use those rules.
I myself now restrict my own options if the codex I am playing tells me to. My Chaos Space Marines gets full access to Iron Warriors relics as does my Dark Eldar Kabal and Guard Regiment, none of which are official colour schemes, but my Dark Angels Successor only gets a single Heavenfall Blade for my relic. If I was to face an army painted as X but using the rules of Y when X has rules I'd ask my opponent to justify that choice. If they were unable to do so satisfactorily I'd request they use the rules their faction has or I would leave the table.
So I ask this: In 8th edition does the colour of your model count for WYSIWYG or not? Would you enforce or be happy to play within restrictions because of the colour of your army?