Salt blocks come in a variety of colors, as any trip to your local feed store will tell you. The specific color of the block will tell you two things about it. One of those is what kind of nutrients are contained in it. The second one is what kind of use it is intended for.
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 depends on what mineral(s) are deficient in your particular operation.