Darker juices gunk coils is a
general rule...it's not
always true

I'm going to complicate things a bit more for you here, sorry... Polycarbonate (very clear) is used most often, and seems to be the most susceptible of the plastics. Polypropylene (softer and tends to look cloudy) can be used with almost any juice without any problems. Some clearos are now made with a combination of these two plastics and stand up to juices better than polycarb alone. The RSST (rebuildable atomizer) was initially advertised as having an acrylic tank, but they seemed to change it more than once after production started. Sometimes, you will see clearos advertised as having improved plastics, but it is tough to know what is in them without some research. But there are some out there that can withstand a wide variety of juices.
Don't worry about the looks of it. It will be easier to tell which set up has each flavor...color coded vaping