If you own a brand and you hover the image of an unbranded horse you will see what your brand will look like on that horse.
This will hopefully make you decide against using a brand that does not fit the horse's colors.

Note that while on the preview the brand is not resized, so on some poses the preview will be larger than the final look.
However, if a brand looks ugly on the preview, it will surely look just as ugly on the final branded horse.