Some of the most common colors of horses you will see are bay, black, chestnut, gray, and palomino. A grulla dun horse is not something you see everyday, as this color is relatively rare.
To get a grulla horse, you can breed a black horse with a dun mare. Though it is not a guarantee the foal will be a grulla, there is a chance it will be.
The most common breed you will find the grulla color in is the Quarter horse. However, the color is still rare within the breed as less than 1% of registered Quarter horses being grulla.