Trying the colour developer by using a B&W developer is interesting to know. If I had some friend developing B&W or if I did B&W myself I would certainly do that. Unfortunately I don't do any B&W at the moment and have no B&W chemistry. Need to sell my house (plug: anybody interested in a house in Rome? Nice price, wonderful quarter) and take a house in countryside and install a proper darkroom. Then I will do B&W.
After careful consideration I decided that I'll certainly take the road of DIY with chemicals when the availability of ready made developer is over (hopefully never). At the moment I only need replacing the first developer and although I see that after all a good scale and a good pH meter would be affordable, I'll live the hassle to another occasion.
Before looking for places where to place an order the question remain to be answered: for "one-shot" rotative processing do I need only the "replenisher"? Or is it better if I buy both starter and replenisher and mix them each time?