How to reduce cracking and increase browning in sourdough loaves?

For the scoring, you didn't score deep enough. You need a lot more depth to allow for enough expansion.

For the browning, you cannot expect much from a sourdough in a home oven. Both the ingredients and the temperature are wrong for a dark crust. The best you can do is to work with washes, milk should brown well, yolk is also OK but not as usual on sourdough.

I'd recommend getting a dutch oven. I use a Lodge cast iron two piece combo cooker. Preheat it in your oven, pop your dough in on the skillet side and cover with the deep pan part. It gives an incredible golden crust!

Yes, the slashes are not deep enough. Slashes are not meant to be solely decorative - they also function to let the dough expand easier.

As for browning... I've baked solely with sourdough bread for the last decade, and I've had no problem at all getting my bread to brown.

I don't use flour for kneading my dough. I use oil on my hands. I'm not a fan of the flour look on baked bread anyway.

I heat my oven up as hot as it can possibly go, and when it's hot, put the dough in the middle of the oven. Then, about fifteen minutes after I put it in, I lower the temperature to around 175-200C for the remainder of the bake. The high heat at the beginning gives the bread better oven spring. My bread is plenty brown when it's done.

If you're still having a problem with burnt bottoms but pale everywhere else, try putting a pizza stone or clay tile in the bottom of the oven to help keep the oven temperatures more even.

The loaf ruptures due to pressure building up after the crust has hardened. This can be decreased by scoring which creates an intentional weak point in the crust. See http://www.thefreshloaf.com/handbook/scoring for more information on scoring bread.

Another way to decrease rupturing is to ensure that sufficient rising occurs before the bread is put in the oven. I typically expect–after shaping–the bread to increase in volume by approximately a third. It will expand further in the baking process as well, but usually only so much as to make the scored marks look appealing.

Regarding a darker crust: I would try modifying your method. Most of your fermentation time is in the fridge. If it is close to 0ºC in your fridge this may stop most of the fermentation from happening (which would also lead to the rupturing above). The fermentation process should breakdown the complex carbohydrates (the flour in your dough) into simple sugars and these will increase the darkness of the loaf.

A couple of things to try: let the loaf sit for an hour before refrigeration (to jump start the process); letting the loaf sit for an hour after it comes out of the fridge; leaving it in a cool place (~15ºC) instead of using the fridge. Try one of these not all of them ;)

The scoring/slashing stuff mentioned in other answers should decrease the cracking.

To improve browning I recommend using a baker's tile. Slip it in the oven before you turn the oven on. It will take you much longer to preheat the oven when the tile is there (my oven takes about an hour to preheat to 450F with the tile) but it has improved the crustiness and darkness of my breads.

This is true whether you bake a dough directly on the tile, or bake it in a pan then remove the pan to finish a loaf directly on the stone.

An added benefit of the tile is that you can cook pizzas directly on the tile, and they turn out a lot better than just pizza on a metal sheet with no tile.

Lack of browning has more to do with chemistry here, than scoring, oven, pans, steam, etc. If the bacteria level of your starter has grown out of control making the pH too acidic, your dough will simply not brown even if it is cooked to death. Also, too much starter and a long fermentation will cause the yeast and bacteria to use up all of the sugars in the flour, hence very little Maillard reaction will result. Same thing if you've over-proofed or used too little salt in the dough. Try a recipe that uses 35g of starter if you plan on the long, slow rise. That way you will achieve all the great flavour of a long fermentation, and also achieve the brown, cherished crust.

You're absolutely positive you're baking at 250 C and not 250 F? It sounds like you're mostly following the same method I do and my bread is usually very dark.

One other potential option-- is your loaf going into the oven with a lot of extra flour on it, which you then knock off after baking? If so, try using less flour, or use rice flour instead. The surface of the dough needs to be exposed to the heat from the oven to darken and extra flour will protect it from the heat.

I think your slashes generally look fine-- the degree of spread that you get in them seems to indicate that they're doing their job. A little deeper couldn't hurt though-- try doing multiple passes rather than pushing any harder.

Other things to try. You can turn the broiler on near the end which will change the direction of the heat from below to above which should help darken the top-- it might end up being too blistery though.

Dutch ovens are great, but if you don't have one you can do a work around by using a cast iron pan and a metal pot with a lid that can sit inside. That'll make your rise better and your crust crunchier.