label changes it color automaticly

i have designed my game with NGUI, and i have some uilabels in that, for some conditions their color is black and for other their color is white. i have attached box collider to it and uibutton script to it as well. it changes color where ever i want. but sometime the color of label is black and mouse cursor is on that label it automatically changes the color to white. so there is no code i wrote on mouse over or something so how can this possible? is there anything regarding tween alpha?

please help