Anything besides what is written in the spell description is up to the DM
It is slippery
The only description we get of the substance from the spell deception is that it is "slick grease" and the rest describe its mechanical effects (difficult terrain, causes creatures to fall prone).
No other properties are defined
Besides the above, nothing else about the substance is defined rules-wise.
In 5e, anything unclear or vague in the rules automatically falls to the DM to decide. This case is no exception. There are no general descriptions of this magical grease anywhere outside of this spell, thus the DM at your table must decide what additional properties, if any, the grease has.
The grease is not flammable unless your DM says it is
As you alluded to in the question, Jeremy Crawford has indeed spoken about the flammability of grease here:
If the grease spell created a flammable substance, the spell would say so. It doesn't say so.
What this means is that nothing in the spell indicates that it is flammable, therefore it is not defined as flammable according to the rules. Something being flammable is something that is a significant property to have for a material and, as such, would be noted if it were intended to have it. This fits with the 5e ethos that spells do only what they say they do.
However, this does not mean that a DM couldn't say that grease, at their table, is flammable.
Even if grease is ruled as flammable, it still requires DM rules to make it work
Great, so, your grease is flammable.
- How big of a flame is needed to ignite it and for how long does it
need to be exposed?
- Now how many rounds does the fire last once ignited?
- How much fire damage does it do and do enemies get a save?
These are all questions that a DM needs to address even if grease is allowed to be flammable. And this is a big reason why the flammability of the grease is a DM decision in the first place. There are no rules for creating grease fires in 5e and nothing in the spell is written as or intended to allow it. Unlike color, or other properties this has significant mechanical effects.
Thus, it is the DM's perview to decide if they want to deal with the mechanic and, if so, how it should work.