This came up when my character was going to use the Assess The Situation card last night. For cards like these and skill checks where the GM doesn't rule that it's a higher difficulty, do they default to a Simple Challange (0 Purple Dice)?

I'm assuming they do, but I just get so used to adding a purple dice to all rolls, unless otherwise specified, because the most common roll is the attack roll.

The GM rules the difficulty of all actions (by adding black or purple dice as appropriate). If its contested, then the default difficulty is based on the opponents appropriate stat and skills, if its uncontested then the GM decides what it is, then the specific action card may add more dice.

As you say the default difficulty for combat is 1 purple, but the GM could decide for whatever reason an attack is more difficult (or even easier than that and actually make the attack simple in some instances), I'm sure you know this already though.

For all other actions, i guess you could say the default is simple, but I would still encourage the GM to add black and purple dice to all these actions (including assess the situation) as s/he sees fit.

Yep, those are simple checks. Those actions are designed to be more reliable, but less powerful, than other actions available to characters.