There are only so many colors you can paint a model, especially if you give them a uniform (of sorts - these
are Orks, after all). Hard to blame folks for drawing the connection when your scheme shares a major element with an established clan. If someone made a custom
SM chapter and painted the armor blue, everyone would assume they were Ultramarines until they were told otherwise, or closely examined the models for details that might hint at an alternate background (shoulder pad icon, divergent wargear/behavior modeled on figures, etc.)
Now, look at homebrew
SM chapters from another angle - how many of them have ridiculous, aesthetically offensive color combos, just for the sake of avoiding the established schemes? How many people play established chapters with lists that run in direct opposition to that chapter's fluff? Paint and play
can be linked quite closely, if the player has that inclination, but whether the guy (or girl... but, come on, let's be realistic, here

) across the table "gets" what you intended is mostly out of your hands. All you can do is give the army your own flavor through modeling or simply explain it, if you really care that much.
Better to have them look good, satisfy your personal vision, and possibly cause some confusion among casual onlookers than to compromise
your army for the sake of that juvenile "I
must be unique, so everyone will recognize my uniqueness"

that so many prescribe to.