As promised, pics of the 98th:


Looks to me like these fellows are wearing 'shirt sleeve order' and shorts.


Couldn't find any pics of the Palamcottah Light Infantry, I'm afraid.
You mention Sikhs in your post. If you want to use Sikh figures then the Palamcottah LI are out, I'm afraid. They were from Madras in south-eastern India and unlike many Madras regiments did not recruit from outside the area. They had Madrasi muslims and Christians, Tamils and Parayans in their ranks, but no Sikhs.
The 98th were a Hyderabad regiment. Though from Central India, they did recruit in the Punjab, the Punjabi Ahir company in the top picture being an example. Don't know the exact make-up of their companies, but it is possible that those two beardy officers in the last picture are Sikhs.
It was the 98th that got famously scattered by bees, so you'd probably want to use them rather than the Palamcottah LI for the "Battle of the Bees" scenario, though they were both present at that part of the battle.
As far as painting goes, khaki uniforms, mid to light brown skin (neither Punjabis nor people from Hyderabad are particularly dark-skinned), brown leather equipment, easy as pie really. Sorry, don't have any Vallejo colours to give you. I've never used that brand.