This part of the casting process will cover what considerations to take when choosing materials, and how to make handling them a whole lot easier. Ultimately, it is up the you to choose the right stuff for you.
I like to get most of my supplies from Smooth-On. They provide everything a person would need.
The hardness of the rubber is measured by its Shore A durometer. I have worked with ranges from 10-90. Depending on your use case, there is a wide range for you to work with.
I recommend always beginning with a 50-60 durometer urethane for casting. From there, you have to judge whether you need something softer or harder.
If you are working on a project that has similarities as other products in the market, try to research the materials and durometer they are using.
There are a variety of pigments that can be added during the mixing process. Many also have additional properties such as UV protection.
Dye isn't necessary to produce parts. Using pigment can be a bit tricky, one drop too many can ruin a cast. This will take some practice to get right.
You might find yourself having to stick some parts together make your prototype whole. Depending on how you expect it to be used, there are various options. One I use often, is Ure-Bond, its great you your protos will be going through some strain and movement.
There are also a epoxies that are available like MT-13 that can bond very well, but doesn't like to be flexed. Also, you can never go wrong with CSA.
Once you begin shopping for materials you will find there are countless additives that could be of use to you. I haven't had a chance to try them all yet.
One I recommend is Kick-It, by Smooth-On. Kick-It is a cure accelerator that significantly reduces the cure time of urethane. I have used it to cut a 48 hour cure time to 6 hours. It makes production and iterating of parts, a lot faster. Keeping rapid prototyping, rapid.
There are also additives to change texture, soften, or harden your materials. Urethane has a short shelf-life, so even getting some life extending supplies are helpful.
Expanding Your Equipment Collection
Urethane casting can be whole skill set of its own. As someone who has to consistently push out high quality prototypes, I need to have the capabilities to produce them in-house. You can get close without, or maybe even just one of these, but with both you can produce some reliable, beautiful parts.
Vacuum and Pressure Chamber
They both serve a similar purpose, removing as many air bubble as possible. I like utilizing both for making parts, but if you are starting off on a budget, I'd get a vacuum chamber first.