Seems like a dubious rumor. Sometimes Watch colors have color-coordinated with iPhones, often not.
Watch Series 5 comes in Silver, Space Gray, or Gold (aluminum and stainless steel), Titanium or Space Black (titanium), and White (ceramic)
Meantime, iPhone 11 - RED, Yellow, White, Green, Black, Blue
iPhone 11 and 11 Pro Max - Silver, Space Gray, Gold, Midnight Green
There was no Midnight Green Series 5 (or RED, Yellow, White, Green, Black, or Blue). There was no Titanium iPhone to match the Titanium Watch...
Would I buy a Watch to match my iPhone, regardless of color? No. To me, they don't seem the kind of things that need to color-coordinate. My Watch is worn, so I do want it to look good on my wrist and alongside my normal (very casual) wardrobe. When I need to dress it up, I change bands (I wear a Sport Loop for everyday, and a Milanese Loop for dress-up). My phone is either in my pocket or my hand. It's in a brown leather protective case, so the color of the phone barely matters.
That said, other people may feel differently. That's the nature of fashion. All that matters to Apple is that people will buy a blue Watch in sufficient numbers to make it worthwhile. I suspect they wouldn't sell that well, so I suspect Apple won't be selling it.