I found the following table in my notes:
US Cavalry - Regular Army
NAME DESIGNATION
August 3, 1861
Seniority Old Name Date New Name Color
1 1st Dragoons 1833 1st Cavalry orange
2 2nd Dragoons 1836 2nd Cavalry orange
3 Mounted Rifles 1846 3rd Cavalry green
4 1st Cavalry 1855 4th Cavalry yellow
5 2nd Cavalry 1855 5th Cavalry yellow
6 3rd Cavalry 1861 6th Cavalry yellow
Maybe this will help.