It just means that they have different genetic makeup or did or did not pick up their parents dominant gene.
Usually the darker genes are dominant, so dark hair, olive skin, dark eyes, are passed down more frequently if at least one parent has them. However, sometimes recessive genes that the parents have match up and two dark haired and dark eyed people will have a light haired and eyed baby if they both carry the light hair or light eye gene.
Here is an example. My husband has dark brown hair, dark brown eyes, and olive skin. I have medium to dark brown hair, lighter eyes which some days are green, some days are brown and some days are grey (no clue why this happens, maybe they just reflect back specific things or something) and pale peachy skin that tans easily due to my Italian heritage.
We have four kids. Our oldest has dark hair, light blue eyes and pale skin.
Our next one has pale peachy skin, blonde hair and light green eyes.
The next has blonde hair, olive skin and dark brown eyes.
The youngest has dark brown hair, almost black eyes, and olive skin.
We are both the parents of all those kids, and our genes just happened to match up that way for each one.
Sometimes if both parents are light or dark and their parents are also both light or dark, they can still have a baby that is the opposite of their coloring. That's can happen if both parents carry the opposite coloring gene from an ancestor. They match up sometimes.
_________________
I'm giving it another shot. We will see.
My forum is still there and everyone is welcome to come join as well. There is a private women only subforum there if anyone is interested. Also, there is no CAPTCHA.

The link to the forum is
http://www.rightplanet.proboards.com