Salt blocks are available to producers in a variety of different colors. This is something any trip to your local feed store can tell you. Each color represents different things about it.
The specific color of the block will tell you two things about it. The first is what kind of nutrients are present in the block. The second is what kind of use it is intended for. Such as the yellow sulfur block or the basic white salt block, there are many different options to go with.
With the different varieties out there it can be quite confusing as to which block is best for you. The one that is best for you will depend on what mineral(s) are deficient in your particular option. Also what you are wanting to accomplish will also play a factor in this decision.