Generally speaking, in most armies ADCs were part of the personal staff of a general so their appointing was accompanied by a great deal of patronage and personal choice. Most armies gave an allowance for ADCs but their basic pay was by the crown as a serving officer.
French ADCs could wear the specific regulation uniform or their regimentals. In both cases they were identified by a coloured armband on the upper left arm – blue for a brigade commander, red for a division and white for a marshal or army commander. (These colours were reflected in the regulation uniform's plume.)
The British army had a regulation uniform for ADCs which was a plain scarlet coat with dark blue facings and a cocked hat. As with the French they could also choose to wear their regimental uniform, an especially popular choice if they were cavalrymen.
Austrian ADCs had a green uniform with red facings and a cocked hat and wore their gold officer's sash over the left shoulder. They were known as 'Adjutants'.
Russian ADCs (adjutants) wore their regimental uniform but with a cocked hat and a gold aiguillette on the right shoulder.
A Prussian cavalry ADC had a smart white uniform with green facings based upon that of the Kuirassiers with a cocked hat. However a single breasted blue undress coat (still with green facings) was more popular in the field. Infantry ADCs wore a version of the infantry uniform with red facings.
I hope that this is enough to get you started but it is a complicated field and you really need to investigate each army. As a start for three of them try here:
centjours.mont-saint-jean.com
goes to sea with the Terrain Mat.
