It's a known fact that dogs do not rely on their sense of sight as much as humans. Why is that? Well simply because people see better overall.
Let me start with an interesting story, that shows vividly how strong dog's sight actually is.
Few days ago we were going down the hill in the forest and he Cheddar ranto some bushes, while I was still going down. Eventually I stopped by the tree and just stared at the beauty of God's creation. Then on the corner of my eye I spotted Cheddar coming out of those bushes, he looked around and then he was climbing up again, clearly looking for me. So I didn't move an inch, neither did I make a noise, just stared at him. He would climb higher and search around with his nose, he clearly could smell me over there. I couldn't stand the tension, as it was really hilarious for me, and let some quiet laughs out. He heard that! Cheddar run down a little and looked around, but couldn't see me, even though it seemed like he totally looked at me. Same thing has happened again - I let a laugh out, he heard, moved down, looked around... Then I couldn't stand it and burst out loud - he ran straight to me.
Just to clear the picture: I was not hiding, just casually standing close to one tree, wearing non colorful clothes, it was around noon and I was in a distance of about 30m (100feet) away and even more at some point.
Let us investigate how Cheddar could miss me, if he is not blind!
1. It has been proven that dogs can recognize objects better when they are moving and sometimes overlook the same object when it is still.
2. Studies have shown that dogs see in colors of various shades of blue and yellow, they can also see best at dusk and dawn. In fact their low-light vision is better than humans.
3. Dogs cannot see as well at a distance as a human, they also have a problem with things that are way too close. On average human can see something clearly as close as 7 cm (2,75 inches) away, compared to a dog that sees things burry if they are closer than 33 cm (13 inches) away.
Remember all that when you're practicing recall, it's not only about being attractive, but also being visible for him.