To achieve a stronger (darker) crust color just increase the amount of malt or sugar.
As for storage of the diastatic malt, like I said, condensation is the big issue. If the package is unopened there is no problem at all BUT you MUST be sure to allow the malt to THOROUGHLY warm to room temperature before opening or you will get condensation on the malt which over time, will lead to severe lumping. Because the stuff is so hygroscopic the condensation will quickly be absorbed into the malt powder, this will be repeated each time the package is opened for use. I have personally seen a near full bag turned into a single block of malt in just a few months, and this was with refrigerated storage (opened for use two to three times a week). The condensation issue is pretty well negated with room temperature storage but remember that each time the package is opened you are allowing more humid air to replace the desiccated air inside the package and the above process continues however at a much slower rate.
The key to effective storage of diastatic malt powder is to minimize the head space in the package, leave it in the original packaging and fold the package down tight to the contents after opening and after each use, secure well using a rubber band and hope for the best. It should go without saying that those living in a drier climate will experience fewer issues than those living in an area with high humidity. The same can be said for summer and winter, with fewer issues in the winter due to the drier air and more in the summer when the relative humidity is significantly higher.
Tom Lehmann/The Dough Doctor