According to the book 'Uniforms and weapons of the Zulu War' Christopher Wilkinson-Latham.
"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. The glengarry badge was of the same design but was made of brass. On 13th Feb 1879 the arms of Perth were authorised to be worn on the glengarry and the tunic collar, but it is unlikely that these badges were worn during the Zulu War".
Hope this helps.