I am a former Hair Stylist, I worked in the business for awhile but decided it wasn't for me.  But I did pick up some "tricks" along the way that I will pass on to you.

If you have ever read the instructions on hair coloring (and you should read them), you will notice it tells you to color dry unwashed hair.  I assumed prior to my schooling and a million hair shows later that if your hair was the least bit damp it would either not work or come out some in some disaster.   Boy, was I wrong.  The reason "they" tell you this is to cause you to use more product.  Without getting into all the chemical reaction ends of hair coloring or dying for the product to work it must throughly saturate the hair.  If your hair is the least bit long or thick and it is coated with dirt and oils and not to mention layers of product (hair spray, gel etc) it will take alot of the color to saturate your hair to get a satisfactory result.  I don't know how many color corrections I have done because someone tried to get buy at home with just one box of hair color. They have areas that didn't take, a spotty look and so on.   Also some times the color is "muddied" or not as nice on dirty dry hair. 

All of the current at home hair color companies make a professional line sold in beauty supply stores.  Yes, Miss Clariol, Loreal, Wella, all of them.  I think not only do they want you to buy more product (you'll notice most of the instructions say that shoulder length or thick hair requires 2 boxes of product) I think they also want you to screw up and have to come to a salon for a color correction (so in the end you buy even more of the product).  Also, some of the first at home hair coloring products were much harsher and relied on your own natural hair oils for conditioning as they were so stripping and had no additional conditioners in them.

The next time you color your hair wash your hair with a gentle bland shampoo and condition it.  Towel dry until hair is just damp (not dripping wet) and then apply your color according to the instructions.  You will notice how much smoother and easier the product applies, and how much further the product goes.  I think you will be pleased and so will your pocketbook.