I looked it up in my Bible. In the King James version, like you stated, it says:

Zechariah 6:2-3

In the first chariot were red horses; and in the second chariot black horses;
And in the third chariot white horses; and in the fourth chariot grisled and bay horses.

In the New International Version, it doesn't mention grisled; the translators changed it to "dappled" instead, just as they changed "unicorn" in parts to "ox". A dappled horse is no more a grisle than an ox is a unicorn!

Now, on to "grisled". This is another form of the word "grizzled".

Grizzled means:
Partly gray or streaked with gray: a grizzled beard.
Having fur or hair streaked or tipped with gray.


This is not the exact meaning in the horse world, however. "Grizzled" is a less-common term for "brindle", a rare horse marking.

A brindle, or grizzled, horse is not exactly a particular color. It's actually a pattern. The horse will have dark streaks over the neck, withers, back, and rump, like somone has drizzled paint or syrup over the animal.

Brindles are very rare in the horse world, and it's possible another color combination could be meant. Roan coloration is also similar to the word grizzled because roans are a solid color whith white hairs mixed in. Other color patterns, such as appaloosa and pinto colors, can also give a grizzly look. Horses that are in the process of graying are often a mixture of light and dark hairs when they are young.

Just going off of terminology, the 'grisled and bay' might mean bay brindles.
If these were brindles of another color, like black or chestnut (red) or buckskin, it probably would have been mentioned. Plus, the usual brindle that most people think of is bay with black drizzled markings.

There also is another type of marking just like brindle, but only the dripped color is white. This may be a form of brindle, or it may be separate. Either way, it's usually called brindle or grizzle.

Because of translations and changes to language, it's hard to know what the orignal word was. It's possible "grisled" either used to mean something completely different, or was a term substituted for a different horse color combination. There are many horse colors that are not solid-- pintos, appaloosa spotting, duns that have stripes, and roans that have hairs of multiple colors, or bay horses in the process of turning gray which might have been considered "grisled".