Brushing your teeth in certain ways without water or immediately following eating acidic foods can actually reduce their whiteness over time.
Skipping the habit of brushing twice a day can turn teeth into bacteria-filled tartar. Over time, excess acid can erode teeth.
Additionally, without regular brushing, food, drink and tobacco stains can bond to the white color of the first tooth, increasing the yellow appearance.
Here is a description of a mistake that can cause tooth discoloration and how to avoid it.