I have been developing B&W films for years. I don't remember exactly when I started developing B&W films, but it has to be when I was in 9th grade. That will be 35 years ago.
What developers I used has slipped my mind except I remember using Rodinal and several other developers. It didn't take many rolls of film before I had reasonable good control on the process. I shot a lot of B&W and a lot of Ektechrome slides those days.
The slide films was developed in a professional lab. A friend of mine who worked there managed to get me a great deal on developing cost. He also supplied me with a lot of deep freezed but expired professional films. They couldn't use them in their professional work so either they were just tossed or they ended up in my hands. Even those days I was thinking of developing slide films myself, but the cost and availabillity of chemicals made it impossible.

Today there are several color processing kits on the market. Both for C41 negative films and E6 color slide films. That removes the excuse of lack of availabillity.
Well, here in Norway it isn't so easy to get your hands on a kit. The shops where you could buy any developer you like in old days are gone. There are just a couple of shops in Norway that carry chemicals. That means that I have to place an order and wait for it to arrive and pay for expensive shipping.

To avoid this I started mixing my own B&W developers a couple of years ago. It started with caffenol, but I found that it was a bit expensive compared to using other developing agents like phenidone or dimezone-s.
The next step was to buy raw chemicals to mix other developers. I bought phenidone, dimezone-s, borax, boric acid, lye (sodium hydroxide) and several other chemicals. The hardest one to find was Dequest 2010 or photoplex-2 as it is named when sold as a photo chemical.

After I had experimented with my own B&W developers over the last two years I decided it was time to have a look at C41 developing.
I bought a couple of Nova Pro Speed C41 kits. Well, I got negatives, but I wasn't impressed. Muddy colors and a lack of sharpness. It was obvoius that they had cut some cornes when they designed the kit. I decided that there was time to mix my own developer to tru to overcome this problems.

Scanning the net for information revealed that there is a lot of information out there. Both correct and incorrect information. Sorting the bad apples from the good took some time, but after a lot of reading and comparing what different people had written I found a basic formula that was almost correct according to all the info I had collected. Adjusting this formula was easy when I had the information about what had to be adjusted.

I mixed the developer according to the information I had gathered and developed a test film. Scanning this film was a bit exciting since I didn't know if I had hit the mark or not, but it seemed to be spot on.

This is an image from he very first film through the developer. Nice clean colors. Red is bright and good as it should be. Red will suffer badly if the formula is incorrect!

Compared to the Nova Pro Speed c41 kit I tried before mixing my own developer, this is far better.

This is an image from a film developed in the Nova Pro Speed kit. Sharpness looks good, but this is from a 6x6 negative shot with a Rolleiflex camera. If this was a 35mm image, the lack of sharpness would be obvoius. What can bee seen here is the lack of brilliant colors. Red is dull and the colors look a bit muddy.

This is also from a film developed in the Nova kit. It is a Fuji film so the colors are a bit better than what I got from Kodak films, but the colors of the sand and the grass still looks a bit muddy. It's also a bit grainy. I didn't expect a Fuji 200 ISO film to be so grainy.

Well, you will get images with the Nova kit, but it is best suited for photographers travelling the world who needs images from their films quickly and don't care so much for quality.

Other kits may produce better quality, but since getting them here is a bit complicated and in fact risky, I didn't want to try any more kits.

Risky? Why? Yes, risky. This because we have to import kits from abroad and the customs here are very strict on testing for drugs in everything we import. One of the components in the Tetenal kit triggers the drug testing kit they use in the import customs.
One individual even got arrested suspected for importing drugs. They had to let him go after several hours of interrogation, but it wasn't a very pleasant experience. Needless to say they tested the chemicals on the lab untill they were so polluted that they were unuseable.

Well, back to the developer.
After making an inventory of what chemicals I had in store, I found that I needed just a few more. Buying them on the net was no problem since I knew where to get them.

Read more about where to get the chemicals and how to mix the developer in the discussions in the group www.ipernity.com/group/c-41-developer