This from the book 'Uniforms and weapons of the Zulu war by Christopher Wilkinson - Latham'.
"At the end of 1878, the 90th Light infantry was concentrated at Utrecht. During the first invasion of Zululand they formed part of Colonel Evelyn Wood's No 4 column which crossed the Blood river into Zululand but after the disaster at iSandlwana fell back on a defensive position at Kambula. During the attack on Hlobane mountain on 28th march, the 90th were left to guard the camp, but the next day, at the battle of Kambula, the regiment played a prominent part. The regiment next took part in the second invasion of Zululand with Newdigate's division in Evelyn Wood's 'flying column'.
The 90th (Perthshire Volunteers) (Light infantry) were dressed as ordinary infantry and had buff coloured facings. It was not until 1881, when they became the 2nd battalion Scottish rifles, that they adopted the unique dark green doublet and tartan trews. The officers' forage cap badge was the stringed bugle horn with the number 90 within the strings, all in gold embroidery. Theglengarry badge was of the same design but was made of brass. On 13 February 1879 the arms of Perth were authorised to be worn on the glengarry and the tunic coller, but it is unlikely that these were worn during the Zulu war.'
Hope this helps.