Colonels and Lieutenant Colonels owe their titles to the Sixteenth Century Spanish King Ferdinand. About 1505 he reorganized part of his army into twenty units called colunelas or columns. These consisted of about 1000 to 1250 men further organized into companies. The commander was the cabo de colunela, head of the column, or Colonel. Since the colunelas were royal or "crown" units they were also called coronelias and their commanders coronels. Later in the Sixteenth Century the French copied the colunela idea and from it developed their regiments in the Seventeenth Century. They kept the title of Colonel and pronounced it the way it looks. The British copied the regiment organization from the French. They also borrowed the Colonel from the French but adopted the Spanish pronunciation of coronel. Why they did is a mystery. The British modified the pronunciation of coronet to "kernal" during several decades of use.
In the French and British armies the Colonels were usually noblemen whose other interests during peacetime or between battles kept them away from their regiments. Also., they had little taste for the mundane activities of drilling, training and marching. The Colonel's assistants--their lieutenants--took over at such times and any other times the Colonels were gone. The Colonel's lieutenants, of course, soon became the Lieutenant Colonels.
Colonels and Lieutenant Colonels led regiments throughout the Seventeenth Century and later and were the obvious ranks for such positions when our Army started in 1775.
Colonels started wearing spread eagles as rank insignia in 1829 when they transferred the gold or gilt eagles that decorated their hat cockades to their collars. Eagles have been popular symbols in our and other military services at least as far back as the Romans. After 1831 most of the Colonels wore silver eagles on their gold epaulettes or gold-bordered shoulder straps. Infantry Colonels were the exceptions. They still wore the gold eagles to contrast with their silver epaulettes and silver-bordered shoulder straps until 1851 when they changed to gold epaulettes and shoulder straps with silver eagles.
Lieutenant Colonels started wearing oak leaves about 1832 on their shoulder straps. The leaves had to be the same color as the shoulder strap borders so Infantry Lieutenant Colonels wore silver leaves while others wore gold. This arrangement, not surprisingly, lead to confusion with some Majors and Lieutenant Colonels wearing gold leaves while others wore silver leaves. The Army did away with that bit of confusion in 1851 by having all officers wear straps with gold borders, all Majors wear gold oak leaves, and all Lieutenant Colonels wear silver leaves as they do today.