Dear Fabricio,
In the 1st picture, the officers have their rank stars on their shoulder straps : this was done to make them less visible to snipers .It became official policy in many Regiments by 1915, though not followed by all officers .
In the second, they have the original arrangement , where the rank stars are on the cuffs , surrounded by pale khaki lace .
Both arrangements were used until the end of the war.
Some officers wore soldiers tunics and equipment when in the frontline , for the same reason, to make them less conspicuous, since the shape of the officers' uniform was different.
In the second picture also, you will see that some officers have small badges on their collars : this was normal in most regiments, though not all . The pattern of badge was different for each regiment.
In the first picture there are none.
The black officer in the first picture is Walter Tull, who is thought to have been the first black officer in the Brtish Army. He was a famous footballer, and served in the 17th battalion of the Middlesex regiment.