When I was about 18, I don't know if I'd even passed my driving test, I cycled up to a small flat roundabout on a bright sunny day. The roundabout had a small tree on it. No cars in sight, plenty of visibility, so I didn't brake and prepared to roll through the roundabout. Only the second time I looked up a car had materialised on the roundabout. I braked in time and there was no near miss, but the fact I hadn't seen it at all shocked me and taught me a major lesson. Had I pulled out I'd have been flattened.
The car was dark grey and had been in the shadow of the tree when I'd looked. I'd been out in the sunshine so my eyes were adjusted to that, and the tree's shadow was like a black hole to my eyes. It's not that I wasn't watching, it's that I had looked, and just not seen it.
So - don't wear dark colours on the road bike, even in sunshine. ESPEICALLY in sunshine, I think. If you really want to be noiticed, wear a colour that's not part of the normal roadside landscape - like red, maybe yellow, blue, or hi viz.