Blue eyes + Green eyes = Brown eyes?

blue eyes + green eyes = genetic eyes

The actual genetics of the inheritance of eye colour is a lot more complicated than the Mendelian model that is taught in less advanced courses.

There are multiple genes that play a role in eye colour, as evidenced by the fact that blue, green, and brown all exist so it couldn't be as simple as dominant B and recessive b.

It isn't all that common, but there are explanations as to why one may have distinct pigmentation that seems at odds with their parentage.

There are eye color genetic charts online. 

It runs in the family. The genetics and inheritance of eye color in humans is complicated. So far, as many as 15 genes have been associated with eye color inheritance. The earlier belief that blue eye and green eye color is a simple recessive trait has been shown to be incorrect. The genetics of eye color are so complex that almost any parent-child combination of eye colors can occur. In biology, an atavism is a modification of a biological structure whereby an ancestral genetic trait reappears after having been lost through evolutionary change in previous generations. Atavisms can occur in several ways; one of which is when genes for previously existing phenotypic features are preserved in DNA, and these become expressed through a mutation that either knocks out the overriding genes for the new traits or makes the old traits override the new one.

That is unusual. Are you sure your daughter has brown eyes instead of hazel eyes? It is important because blue eyes is the result of a total lack of the pigment eumelanin (dark brown to black) in the eye. Without eumelanin, blue light that enters the eye will not be absorbed but is instead scattered (in the same way that blue light from the sun is scattered by molecules in the atmosphere to make the skin look blue. 

Green eyes is the result also of a lack of eumelanin in the eye. However, green eyed folks have a small amount of phaeomelanin in the eye. Phaeomelanin is what make the hair look yellow or red. Lots of phaeomelanin = red hair. Small amount of phaeomelanin = yellow hair. For the same reason, a person who would normally have blue eyes would have green eyes if he/she has a small amount of phaeomelanin in the eye (since yellow + blue = green). However, if there is lots of phaeomelanin in the eye, the result is hazel eyes, since the phaeomelanin will absorb the blue light if there is a lot of it. So, look more closely at the eyes of your daughter. They should be hazel, not dark brown or black since that there must be eumelanin in the eyes for them to look dark brown to black. Blue eyed folks have genes that result in the inability to make eumelanin, so it is rather rare for a blue-eyed person and green-eyed person to have a child with dark brown to black eyes. 

Since it takes multiple steps chemically for the body to produce eumelanin, blue eyes can be caused by mutations of several different genes. For example, if A, B and C are precursor molecules for the manufacture of eumelanin, people would need, say, genes A, B and C to make eumelanin, A B and C being enzymes that are needed to complete the chemical process. Let's imagine someone has recessive genes that result in the inability to make eumelanin aa. Another person has recessive genes bb, and that person also cannot make eumelanin. Both have blue eyes or green eyes. However if they produce a child, then the child will have a genotype of Aa and Bb, The child, because it has a good copy of both genes that are needed to make eumelanin, will have brown eyes. I suspect that may be why your daughter has brown eyes, if indeed she does have brown eyes instead of hazel eyes. 

Passing on genetics is not an addition, not a mixing of paints. It's a lottery.

There's no way to calculate or predict which half of your genetics you pass on to your child.

The idea that eyecolour is as simple as assumed has been proven wrong, just like the assumption that humans come in races, or the assumption that the alleles for bloodtypes A and B are never found on the same chromosome (and its implication that at least 10 % of the population is not begot by their legal father...)

There's more than one or two gene loci which help determine eye color in humans.  It's not a simple Mendelian situation.  Teachers and textbooks should stop using human eye color for sample problems because they don't fit the real world.

Next thing you know, your daughter is going to be asking you if you can roll your tongue.  That's in textbooks but not a real world situation either.  Tongue-rolling is a learned behavior; it's not genetic at all.

Anyway, if you have somewhere between $300 and $600 lying around, you can get DNA tests for you, your husband, and your daughter.  Google for

DNA testing