All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The forbidden imitation of the non-Muslims is in things that are peculiar to them either in regard to clothes or else. As regards what is generally worn by Muslims and non-Muslims, then this is not prohibited as clarified by the scholars ; for more benefit, please refer to Fataawa 119486 and 88123.
As regards the uniform that Khalid Ibn Al-Waleed used to wear in the war, then we have not come across any text which states a special uniform that he used to wear, but he used to wear what was suitable for the battle.
However, there is no great benefit in knowing this: clothes and the like are among the matters pertaining to the custom. In addition to this, what is useful for them at that time could not be useful at out present time especially in regard to the military uniform as the military weapons and the nature of wars have changed.
Allaah Knows best.