You might wonder, "What causes fossils to have different colors?"  This is so complicated that most people won't even try to explain it, but here goes my attempt.  Please excuse my oversimplified information.  This is just a starting point.  If you are interested in geology, there is SO MUCH more to learn.

First, a fossil is no longer the organic item that it once was.  It has been "mineralized."  The organic material has been largely replaced by inorganic minerals, leaving a nearly exact replica of the original item.  For example, a shark tooth fossil is no longer a shark tooth - it is a rock.

So what is a mineral?  It is a basic element (such as carbon), or more commonly, a compound created by the chemical combination of two or more elements.  An example is iron oxide (rust), which is the combination of iron and oxygen.  There are thousands of minerals and each contributes unique color qualities to the fossil.  The color of the fossil can therefore vary GREATLY, depending on what minerals were used in the "replacement" process.  Below are some common fossil minerals.  This list is not complete and the displayed colors are not exact.  This is just a place to start your research.

And of course, just like mixing paints, you can achieve other interesting colors by combining different minerals.  Or the fossil may contain rare minerals, producing more exotic colors.  And in addition to that, different parts of the fossil may be composed of different minerals, generating interesting patterns. 

NOTE:  I haven't even begun to explain how different conditions (moisture, heat, pressure) can alter a mineral's color.  If you really want to understand this, grab a good book on Geo-Chemistry.  If you have difficulty sleeping, reading this book has the additional benefit of inducing sleep.  Also, it will work well as a door-stop or booster seat.

I found this unique tooth purely by accident.  We often find damaged teeth and I wasn't particularly wanting another.  This one turned out to be very special though, for two reasons.  First, it is a colorful tooth, with several different minerals represented.  Second, when I turned it over, to my surprise, every bit of enamel had been peeled off the back of the tooth.  This revealed the composition of the underlying material, which was once bone and is now rock.  As you can see below, it is composed of "clumps" of different minerals, each with it's own unique color.