Years ago I used a period Pharmacopoeia Formulary book, that included details of what form and appearance each respective medicine was suspose to looked like, and in what form it generally came in.
Also had a couple friends that were pharmacutical chemists helped a bunch too. Will have to do some digging to find my old research lists for it. To see if it might add further fodder.
In most cases where where the product was in a crystalline (a salt) form, one could use Salt (Sodium Choride), Alum, Citric Acid, Epsoms Salt, or Rock Salt, depending on how fine or course grained the original product was. I generally avoided anything sugar based since it tends to rapidly absorb any residual moisture and can turn into a solidified brick in your jars. Most of the salts can sometimes clump up but you can generally shake them loose again.
Powders. One could use Cream of Tarter, Baking Soda, Corn Starch, etc for starters. If memory serves correctly raw form Opium was a fine black powder. For this I used dehydrated activated charcoal.
Herbal items one can generally find a close match going through the spice/herb racks
If one is making items in pill form it gets a bit tricky sometimes. Most times one can use flour made into a stiff dough, can add food color if the product had a particular color hue desired, roll/form into pill sizes you need and let dry. They tend to hold up fairly well. I still have some that have been in jars 20+ years and still look like the day I dumped it in them.
Recommend avoiding any product that is or could be caustic, volitile or reactive in itself, or to anything else present, if something were to break or spill in transit. Also caution on products that could freeze and rupture the given container, if displayed or stored in areas subject to possible freezing temps. These items mentioned intended solely for static display purposes only. I had other items I used for demonstrations, sick calls and surgical demos.
For pills, tapioca pearls work fine ... just spritz them with food coloring ... but don't go bananas with the food coloring ... you don't want your quinine pills looking brighter than the sun, LOL!!! Opium pills, I use peppercorns ... they are as about as close as I could find. If you use something that isn't consumable (say like face powder for calomel), make sure it is secured, so "lil Johnny" doesn't pop the top, thinking it's edible, and start swallowing.
If this is for a field station impression ... all I use are morphine, opium and quinine look-alikes (fine sugar for morphine, peppercorns for opium and yellow food coloring spritzed tapioca pearls for quinine pills). You wouldn't really need anything else "in the field". Remember, the care you're giving, is only temporary ... it would be enough to stablize them until they arrive at the field hospital.
One also should keep an ingredient list for each "medicine", have it on hand somewhere discreet... just in case the nice modern Police Officers take a dislike to your driving style. In such a manner, the Police Chemists can compare your faux medicines with what you say they should be... see, Officer... baking soda and blue lake #2, not powdered sudefed at all. 0:-)
A bit of anise or licorice extract will give your "opium" a more realistic smell, "laudanum" can be a licorice liquor of some sort if you want to carry some in a pocket flask. I added benzoic acid to the pills I made years ago so they wouldn't mold but still not be unsafe. I like the idea of peppercorns, it would be much easier to use them than to roll pills and with all the essential oils available you could make them smell like anything you wanted.
I was wondering, what is the most effective way of adding color to the salts or powders if needed?
I used ground up school chalk for the colored powders and as others have mentioned tapioca pearls spritzed with food coloring for pills. Along with pepper corns, corriander seeds etc. Simple salve can be a cheap cold cream and mercury salve the same with a little blue food coloring.
Also I played with the colors on my pills, if you are not careful they will look like skittles...but lightly spritzed they look tinted not covered. I made mine in small batches and colored them then rolled them around on paper towels. Then heated in the microwave 10 secs at a time and rolled between heatings (first batch I didn't do this and they stuck together in a large mass). These have been made for several months and have no signs of breaking down or mold.
The other items I duplicated with water and food colors, even my whiskey just in case some one got their hands on it. Nothing in my meds are what would be considered harmful for consumption.