Could a women wear a white dress to a wedding if its just a simpe sleak dress?

No, no, no and no. White at a wedding is usually only reserved for the bride no matter how simple or sleek the dress is. It's just proper wedding etiquette not to wear white.

Could she, yes. I've just always considered it bad form to wear the same color as the bride on her wedding day. I've always thought of it as showing her respect.

I say to hell with etiquette.

Do you realize that one of the dresses I regally wear as part of my daily wardrobe, actually IS a wedding gown (big bouffant skirted "Dallas" type from the 1980's).

Wear what ever you want, whenever you want, don't take any slack about it from anybody.

NO, that is extremely poor etiquette.

The only person that should be wearing white is the Bride.

No white for you!! Its the brides day for white... even if she isnt a virgin

Not unless you hate the bride or something, for instance if it's your s/o's ex-girlfriend. But the rule of thumb is NEVER wear white to a wedding unless it's your own. No variant of white either (off-white, cream, dove, you get the picture) ;)

You could...
But I wouldn't advise it.
Its the brides day, you really shouldnt wear the same colour as her.
It'll bring unwanted attention and possibly unwanted bad responses from people.