Oldboy,
Umra Khan has already given you great advice, and he is a wonderful fellow, not only generous but also a devoted follower of the True Path when it comes to colonial wargaming rules! Nonetheless, I'll expand a bit on his excellent guidance...
Re: Anglo-Indian army turbans, in addition to the regimental color and/or pattern, there is the more basic question of the type of turban itself. As miniature gamers choosing figures we often don't get much choice in this matter, but in the past half decade or so more variations have appeared for sale from companies like Perry Miniatures and Artizan, so it may be worth taking note of this handy chart:

Moving on to regimental color patterns, the closest thing to a single dependable source is the pair of volumes Umra Khan mentioned above: C.Y. Carman's
"Indian Army Uniforms, from the 18th Century to 1947, Vol. I: Cavalry" and
"Vol. II: Artillery, Engineers and Infantry."These are very useful reference books and have entries with uniform information for every regiment, but it isn't focused on the appearance of units in action during particular campaigns. You will find specifics on some regiments during some wars, but little or none on others during the same or different wars. Still, at present it remains the closest thing to a single authoritative source.
I own both volumes and never regret having bought them, but there are some other, less expensive books that can also be useful. At the top of this list for me are 2 Osprey Men-At-Arms titles, both first published in 1979 but still in print and easily available:
"Bengal Cavalry Regiments, 1857-1914" (MAA 91); and
"Indian Infantry Regiments 1860-1914" (MAA 92). Again, when it comes to uniforms and equipment of every regiment in every campaign of the time-periods in their titles, neither of these are all-inclusive, but they do have a lot of useful information, including turban patterns for many regiments.
Another less expensive book is,
"An Assemblage of Indian Army Soldiers & Uniforms by the late Chater Paul Chater," published in 1973. Chater was a talented artist who did color paintings of Indian soldiers from a variety of different regiments. Again, this is nowhere near comprehensive, but virtually every page features a full color painting of a different uniform for a different Indian regiment, turbans included.
Umra Khan also mentioned a great little book by A.H. Bowling, published by Almark in 1971, called
"Indian Cavalry Regiments 1880-1914," which in addition to contemporary photos, includes one small color illustration of virtually every regiment of Indian cavalry pictured at some point during the late 19th or early 20th Century, turban patterns included.
Almark published another great book in 1979, written & illustrated by Christopher Rothero, called
"Skinner's Horse," which has tons of visual detail on different uniforms and turban patterns worn by the 1st and 3rd Skinner's Horse throughout their long histories, including weapons and lance pennon patterns.
Helion recently published a large format book including many color uniform plates, called
"The Regiments of the British Indian Army 1895-1947." Based on the title you can see it's mostly focused on a later era but it does include some late 19th Century uniforms and turbans, and is easily available here:
https://www.helion.co.uk/military-history-books/the-regiments-of-the-british-indian-army-1895-1947-the-indian-army-of-the-crown-in-colour-paintings.php?sid=f8a66ba8c8f6ec84adc372469e1e3477The two most famous illustrators of the Anglo-Indian Army are probably
A.C. Lovett (who was a serving officer in the British Army) and
Richard Simkin (who some say was a part-time soldier in the Artist's Rifles). Lovett's
"The Armies of India" (1911) includes tons of uniforms and turban patterns, some of which fall within our period. If you can get a copy, fantastic, but if not, you can Google around and find some of the images from its pages:
https://www.google.com/search?q=lovett+the+armies+of+india&client=safari&rls=en&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjHhIupmOHyAhVGwZ4KHTGHDg0Q_AUoAXoECAEQAw&biw=1552&bih=898The same goes for Simkin. Like Lovett, some of his illustrations post-date our period, but others can be useful. Here's a LINK to a page featuring 21 of Simkin's Indian uniform prints:
https://www.military-prints.com/indian_regiments.htmRe: Anglo-Indian regimental flags... I may be wrong -- in fact I hope I'm wrong! -- but unfortunately I don't know of any book devoted to the subject. If you Google around online you can find some contemporary photos and color illustrations featuring flags of a few different Indian regiments from this period -- including 1st Bengal light Infantry, 8th Bengal Native Infantry, 15th Ludhiana Sikhs, 16th Punjabis, 29th Bengal Native Infantry, 30th Punjabis, 62nd Punjabis, and 80th Carnatic Infantry -- but in my experience that's about it. Rick O'Brien The Flag Dude does indeed have awesome miniature flags for some Anglo-Indian regiments, including the 1st Bombay Grenadiers and 30th Bombay Native Infantry Jacob's Rifles, both of whom fought at the 1880 Battle of Maiwand.