TL:DR Yellow (or Gold) Storylets are part of a larger story arc, so it might be more interesting to pursue them, if you can. If not, try some plain white (or cream) cards to build up your Qualities and Traits.
As far as I can tell, there are four different types of Storylets, each with their own colour (note that I present some out-of-context examples, which might be considered slightly spoilery):
- White Storylets are the most common, and can be repeated as many times as you'd like:
- Bronze Storylets lead you on a short story with a distinct conclusion, but it seems that they can also be repeated:
- Gold Storylets are also part of a larger storyline, but a specific Gold Opportunity card or Storylet may only be completed once:
- Blue Storylets are part of an Ambition storyline, which are large, overarching Storylets that are supposed to span your entire journey in Fallen London (an exception might be the Knife-and-Candle Storylet, which is also blue, but isn't really an Ambition in the strictest sense):
Side Note: Some white Storylets act somewhat like gold Storylets, in that they are only playable once and progress along a larger story. A way to distinguish them from normal white Storylets is that these type of Storylets seem to require a very specific quality, and when completed they will advance that quality.
Here is an example of the Storylet I'm trying to describe: