This has happened to Ace twice. The first time he had something in his eye which came out on its own and the problem cleared up. The second time was quite recent and he had a mild infection. The vet gave him some ointment that had to go on the eye twice a day for five days and the problem cleared up no problem.
Definitely depends on the color, and to a lesser extent, the texture. We battled a good two month war with eye infections with Iorek and the difference between his infected eye boogers and his normal (generally allergy induced) eye boogers is undeniable. When it's infected, it's very green or sometimes yellow. It's persistent and I would find myself wiping his eyes every time I saw him. The texture is very snotty and runny and had a tendency to "pool" in the corners of his eyes. Even if I wiped away the outer boogers, it seemed like it never ended from the corners of his eyes. His eyes just generally looked unhealthy...red, itchy, wet etc.
Normal boogers for him vary from being totally clear to kind of just a dirty, dusty color. The clear indicates allergies and they are pretty sticky/stringy because his eyes are dry. The dirty/dusty color tends to happen after lots of play and running around and getting gunk in his eyes. Those tend to be crusty. He'll have some boogers when he wakes up and maybe some at the end of the day but that's it. He can go days without any boogers at all.
Eye drainage is always good to get checked out if it seems more persistent than it should or if it seems like Percy's eyes are bothering him. It could be allergies, it could just be normal drainage, it could be dry eye, it could be an infection... Unless you feel super comfortable with your dog's personal eyes and recognize the changes and what causes them, it's generally best to let a vet take a peak to rule out anything significant.
Has his eyes ALWAYS drained? Do his eyelids "roll" inward(entropion, ectropion)? Does he have extra eyelashes on the inside rim of his eyelids (distichiasis, trichiasis) that could be rubbing on his eye? Entropion and Ectropion can be very severe and cause damage to the eye and loss of vision. Distichiasis and Trichiasis severity depends on number of hairs involved and if they are soft or stiff. These can cause mild drainage to scarring and vision loss. These hairs can also come and go as well so they may not always be present. Good luck