Depending on the colour of your tap water, the cause could be a number of things. The most common colours seen are:
White/Cloudy: Extra air trapped in your water as it moves through your pipes. You can tell if the tap water becomes clear in a glass after a minute or two. This tends to happen when water pressure is high and should be checked out to avoid future plumbing problems.
Yellow/Brown: Dirt and naturally occurring sediments can settle at the bottom of supply lines despite the water treatment process and get shaken up when water passing through the pipes speed up due to a water main break, or high service demand.
Orange/Brown: If your house uses cast iron pipes, they will corrode over time. As this happens, tiny iron and manganese particles fall off the inside of the pipe as water passes through it. This is a sure sign that the pipes need to be replaced.
Visible Black Particles: Rubber materials such as gaskets and O-rings begin to break down as they get older. As water flows through the pipes, pieces of these rubber parts can break off and end up coming out of your tap. This is another sign that plumbing replacement service is needed.