Textbox background colors

Have been coding a number of asp.net web enabled applications and when entering a textbox or series of text boxes, even though the background colors have been set to white, after compiling and running the code, the textboxes appear yellow. There doesn't seem to be any pattern, I don't code these with a default value or make them mandatory and even going into the properties of the textbox and changing the background color to white doesn't help at all, the text boxes still appear yellow. A good example of this is in this forum's registration procedure. How do I stop this?

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