Although horses come in many different colours (depending on the breed). There are only two colours that all other colours are built on. These are Black and Chestnut. 

When determining the colour of a horse we do not consider white markings - if a horse is silver dapple pinto, then his colour is Black with a silver dilution gene, his pattern is pinto.

If we got rid of all Modifying and diluting genes and all white pattern genes we are left with red or black.

If we get technical...

The Extension locus is present in all horses and this is the deciding factor of the base colour of a horse, 
which will be either black (E)  or red (e). 

Black (E) is dominant - if a horse has this dominant gene it WILL be seen.
Red (e) is recessive - this means that it can be "hidden" by a dominant gene (ie black).

So in other words, all horses have the Extension locus with three possibilities:

• EE means the horse is genetically (homozygous) and physically Black*

• Ee means the horse is genetically black and red, but physically Black*

• ee means the horse is genetically and physically Chestnut*

*These look Black or Chestnut when there are no dilution or modifying genes present.