Canon is a funny thing in Warhammer. The official literature isn't so much there to define what you as the player can do with the setting as provide soft limits. There are hundreds more Space Marine chapters in the background than have ever been named, endless unidentified worlds raising endless undefined Imperial Guard regiments. Gamers and hobbyists have spent three decades now slotting their own ideas into those gaps.
We've all done it, whether we're creating a whole new Space Marine chapter or just our own Captain for an existing one. Within the confines of our own gaming groups canon is ours to define using the official literature as a foundation.
Which is the long way of saying that sometimes, just sometimes, we can just turn around and call the official, GW-sanctioned background wrong. For instance, no power on Earth can convince me the Grey Knights would ever accept technical support from an alien orang-utan when there are perfectly good tech-priests about the place. And then there's these:
These are the official unit markings of Black Templars squads. As far as I am concerned only the top two are correct. The reason this comes up is I recently inherited some kits from fellow gamers: a half-depleted Black Templars Upgrade sprue and an untouched box of Sternguard, plus an old metal Black Templars Marshall. I've always liked Black Templars so I thought I'd make a single unit out of Sword Brethren out of the Sternguard, maybe use them as a detachment for my Imperial Guard some time down the line.
Then I looked in the Space Marine Codex and saw that Sword Brethren had black and red markings. No. No no no no no. Black Templars have black iron crosses on a white background. All of them.
This isn't just petulance on my part. Mostly, but not entirely. What got me into Warhammer 40,000 were the Index Astartes collections and, in particular, the Black Templars article...
… so my first image of these Marines was the very stark black and white aesthetic and anything else just seems wrong to me. I'm not saying GW don't have the right to expand the background however they like and I'm not saying anyone is wrong for painting their Sword Brethren with red insignia, simply that as canon is largely ours to define there's a certain freedom to use our own definitions of what's right if we disagree with something.
Of course, I am forcing myself to paint white here so maybe this freedom isn't all its cracked up to be.