Depends on your eyesight and daily conditions.
Like many people, I see bigger, darker rockets easier. For me, white, gray, and any blue other than dark blue turn into sky above about 2-3K feet. Anything smaller than a Leviathan disappears above 2500 feet.
I thought that silver and reflecting tape would disappear, too, but every silver rocket, regardless of size, flashes nicely while coming down. Even with cloud cover, reflecting tape works. Heck, even glossy paint will flash nicely in the sky and make tracking easier.
Green turns into sod/corn/beans on the ground! :wink:
Tracking smoke is your friend :shock:
4" diameter 6 foot tall rockets that are painted a dark color go WAY up before becoming invisible. Things I launch that are going high and/or fast I try to do in large sizes or dark colors.
For parachutes, I make mine in alternating light and dark colors for contrast against the sky and the ground.