וּבַמֶּרְכָּבָ֥ה הַשְּׁלִשִׁ֖ית סוּסִ֣ים לְבָנִ֑ים וּבַמֶּרְכָּבָה֙ הָרְבִעִ֔ית סוּסִ֥ים בְּרֻדִּ֖ים אֲמֻצִּֽים׃
and in the third chariot white horses; and in the fourth chariot grizzled bay horses.
Read kabbalistic (mystical) interpretation of Zechariah 6:3: and classical mystical sources.