It is best to use a container that lets no light in when using any chelated iron in solution, but one alternative to using an opaque container is to just make really short lasting mixes (say, a week to two weeks) and keep the bottle in a cupboard between dosings. The other trick here is to make such a dilution with big doses to make your life easier and lower your margin of error when using a target.monitoring based dosing system like PPS: make your mix for doses of say 15mL (1 TBL) or more.
But like anything you can makeshift a workaround with the opaque bottle thing, of course. For example, when I used to reuse the Tropica Master Grow bottles (which is the same traditional two chamber dosing bottles as linked by Brilliant), I ripped the bottom off a ginormous cozy I got from Vegas and slipped it over the bottle. Tape works. If someone who loves you knows how to knit, those kinds of little cozys can be cute.
DTPA and EDTA Fe exposed to light/allowed to go to Fe +3 will fall out as a brown precipitate. However, if using CSM+B I would first try making a mix using DI or RO water. There is always some stuff in CSM+B that doesn't fully dissolve, and for this reason its better to use it in the type of bottle you bought (or the traditional two chamber setups) than, say, the stuff fishyface and I are interested in. Right now I squirt out of a colored ketchup thingy like you find at a fast food joint. Lots of other trace mixes go close to full solubility after good shakes though (ex: Miller Microplex).
(Though I should state I have never used regular CSM or made it CSM+B. I have only tried the "CSM+B+Extra Fe" mix Greg Watson made a few years ago -- 2004? -- for about a year then it sat for a while and now I still use it sometimes.)
But lots of folks don't take these things that seriously. But anyone can see the Fe+3 percipitate thing with most any trace mix and a sealed container out in sunlight. Its just that many commercial bottles are opaque and say shake well first
If you're more interested in this, here's a good study Clemson did with easy to understand language that will get you started: http://www.clemson.edu/hort/sctop/bsec/bsec-04.php
Note claims of 85% of chelated Fe becoming unusable Fe+3 after 10 days when exposed to radiation (light) but 0 loss when in darkness.