What is the significance of white horses?
Rashi: They drove the chariot of Paras, which build the Beis ha'Mikdash.
Radak: This refers to Yavan. I do not know why they are called white 1 . Rav Sadya Gaon says, it is because Yavan's first king was Alexander Mukdan, and he was a wise philosopher, and Chochmah is white and clear.
Malbim: After Romi ceases, there will be another kingdom that will not spill so much blood, and will not harm Yisrael so much. The Galus will be lighter in the four directions of the world, like it is in this millennia. The horses of the chariot are white, and bestow Chesed.
Radak holds that the kingdoms are in chronological order, therefore it must refer to Yavan. (PF)
What is the significance of Berudim horses?
Rashi: They are spotted, to drive Yavan, which afflicted Yisrael with different kinds of decrees.
Radak: Berudim have white blotches, like Barad (hail), amidst another color. This is appropriate for Yishmael, for they think that they guard Moshe's Torah, which is white like hail. However, they mix it with many other beliefs, just like hail is mixed with black or another appearance.
Malbim: This is the kingdom that will rise close to the time of the Ge'ulah. Refer to 6:6:3:3.
What do we learn from "Amutzim"?
Rashi: It is to drive Edom. I do not know what is Amutzim.
Rashi citing Targum Yonasan: They are Kitmanim. Radak - their appearance is like ashes. This hints to Edom, which lowered Yavan, and it ruled. It has two colors, for another nation rules with it, i.e. Yishmael. So it says in Nebuchadnetzar's dream, like Daniel interpreted it - it will be a divided kingdom.