You recently mentioned in Reply 680 at the Lehmann preferment thread at http://www.pizzamaking.com/forum/index.php/topic,9908.msg119554.html#msg119554
that you were thinking of giving "the naturally leavened doughs a break for a couple of weeks". That comment got me to thinking about the effects on crust coloration of of the naturally-leavened milk kefir (and Ischia) poolish versions of the basic Lehmann dough formulation you have been using.
To frame the issue, I went back to the Lehmann preferment thread where we first started to talk about taking pH readings. As best I can tell, we started talking about pH readings fairly late in the game in the Lehmann preferment thread, at Reply 626 at http://www.pizzamaking.com/forum/index.php/topic,9908.msg112151.html#msg112151
. By comparison, we have been talking about pH readings in this thread from about its beginning. Since you were having fewer problems with crust coloration with your Lehmann preferment doughs, I tried to compare the pH readings for those doughs against the readings you got with your milk kefir poolish Lehmann doughs. What I observed as a trend is that the pH readings for the milk kefir poolish Lehmann doughs were noticeably lower than the values that I saw for the Lehmann preferment doughs. For example, the range of values for the milk kefir Lehmann doughs were around 4.44-5.42 whereas the pH values for the Lehmann preferment doughs were around 5.40-5.75. Moreover, and you can correct me on this if I am wrong, but it seems that the more milk kefir poolish you used in relation to the total formula flour weight, the lower the pH values became.
The above again raises the specter of the relationship of pH to crust coloration, as reflected by the residual sugars and the like. You might recall that there have been several discussions on this matter, with several posts directed to the subject, including those at Reply 2 at http://www.pizzamaking.com/forum/index.php/topic,9635.msg83546/topicseen.html#msg83546
and at Reply 155 at http://www.pizzamaking.com/forum/index.php/topic,11700.msg111250/topicseen.html#msg111250
. In your case, your pizzas did not seem to suffer from insufficient oven spring that Prof. Calvel warned us about but, unlike Prof. Calvel who talked about the subject in the context of baking bread, you have been baking your pizzas at oven temperatures considerably higher than what would be used to bake bread.
As I pondered the above matter, I also recalled that Tom Lehmann mentioned somewhere the effects of large amounts of organic acids on crust coloration through caramelization (and, I assume, the Maillard reactions). It also seemed to me that he recommended using dried dairy whey or non-fat dry milk powder in order to get better crust coloration. It took me a while to track down where he addressed the matter but I was able to find it at http://www.pmq.com/mag/2001spring/lehmann.php#1
. Note, in particular, Tom's comments on the buffering effects of calcium on the acids and this effect on crust coloration. Tom also talks further about the use of dairy whey in the third question at http://www.pmq.com/mag/2006april-may/lehmann.php#3
. We have talked about using dried dairy whey before but deferred actual use until we saw how other possible solutions panned out, including using diastatic malt, more sugar, honey, etc.
You might want to consider trying either dried dairy whey or dry nonfat milk powder to see if they offset some of the acid components produced by your currently high level of usage of the milk kefir poolish. No doubt your milk kefir poolish have been working well to produce good crust flavors and textures but, at the same time, they may be producing too many acids leading to low pH readings and, possibly, problems with crust coloration. If using dried dairy whey or dry milk powder do not improve matters, then that might mean having to take a fresh look at your basic milk kefir poolish/dough regimen to see if it can be improved. I previously mentioned going to a sponge approach, which I believe would slow down the overall fermentation process.