In general, nouns can be divided into abstract concepts and real-world (concrete) objects. Furthermore nouns can be divided into things measured by quantity or degree, and things measured by number.
As a rough principle, abstract concepts (like love or peace) and nouns measurable by quantity (like water or sand) tend to be uncountable by default. Meanwhile, concrete nouns measured by number (like lions or buildings) are usually countable.
Unfortunately there are far too many exceptions to this to make it completely reliable. For example, one can have many loves in one's life. Fish can be counted by individual animals, or by some uncountable amount of weight or volume ("a full net of fish").
What does seem to work, at least with some consistency, is to find the closest synonymous noun that you know to be countable or uncountable, and use that as a guide. For example, let's say you come across a nifty noun like senescence, which means "the deterioration that comes with age". It's an abstract concept, and probably measured by degree, so you can tentatively assume it to be uncountable. Since you also know that the relatively similar noun crazy is uncountable, you can be pretty sure that senescence is also uncountable. Example:
Even as the greedy king's aging body slipped into senescence and decrepitude, his desire for more life burned ever hotter.
As a counter-example, take an abstract noun like plutocracy ("government by the rich"). Yes, it is abstract, but at the same time you know the noun government is both countable and uncountable. You would not be wrong to assume that plutocracy is the same -- countable when referring to specific instances:
With the current state of campaign finance laws, it would not be an exaggeration to call our current system of government a plutocracy (by proxy), rather than a republic.
And uncountable when referring to the abstract concept:
Plutocracy is the default state of any government that lacks a strong national Constitution.
Your examples mood and freedom can be determined the same way. Mood is like feeling, while freedom is like value. Feeling and value are both countable and uncountable, so you can tacitly assume mood and freedom are the same.
Specific instance:
Mom is in a good mood today.
Yoga gives me a freedom of movement that I don't get from other kinds of exercise.
Abstract concept:
The other captains say that whether you can make it through the Devil's Triangle is entirely up to mood and chance.
What good is bread without freedom?
Nevertheless, because there are so many irregularities, you're likely to have to memorize more than a few, and be corrected when you make mistakes. Even native speakers learn these by reading, remembering, and regurgitating.
For example, suppose I ask you to go to the store with a shopping list on which is "steak". You need to go to the butcher and ask for steak, and you know meat is uncountable, so you assume steak is also -- until you hear the person ahead of you ask for "three large steaks". So you assume it's countable. Then the butcher asks you "how much steak" you want and you realize it can be either, depending on whether you are measuring by weight or by number of portions.
Now you have steak in your set of known nouns, so if you come across another meat-related noun like brisket, you can assume it's similarly uncountable when measured by weight
I need three pounds of brisket for this recipe.
and possibly countable by portion
Dad always cooks a brisket for Sunday dinner.
(Edit) To be clear, when native speakers first encounter a new noun, it's with enough context to specify it as countable or uncountable. Again, to pull out a fairly esoteric example, replevin, a legal term meaning "a procedure to recover unlawfully seized property". Few (if any) who haven't been to law school will know or need this term -- but those who have, will have seen it in context written something like
the law and practice of replevin
or
a writ of replevin
and this is how they will use it, as a phrase rather than a stand-alone term. They wouldn't say "a replevin" or "replevins" -- even if they dictionary says it's OK -- until they see someone else us it that way.
Best Answer
Many “uncountable” nouns can be used in a "countable form" when they refer to a broader meaning. The notion that using an s to pluralize an uncountable noun must be an error is a trap.
Words like waters, rices, sands, and oxygens are valid plural forms, although some of them are quite rare because you would only use them in unusual contexts. For example, a scientific textbook reads:
I’m reminded of a debate that raged over at ELU some years ago about the plural of equipment. The accepted answer was downvoted and drew more than 30 comments and has since been deleted; it read:
Fact is, the use of equipments is uncommon, not prevalent, and it’s not necessarily wrong or ungrammatical in those usages.
You’ve uncovered an excellent example with the word paints. Most of the time, paint can be used to even when referring to a plural. For example, if I go to the store and buy a gallon of yellow paint and a gallon of red paint, I would probably say:
(not, “I bought paints today.”)
But there is nothing wrong with me saying:
In that case, I might be talking about how there are oil-based paints and water-based paints, latex paints and acrylic paints, matte paints and glossy paints, interior paints and exterior paints, etc.
I think what you say about Macmillan is on the right track, but off just a little bit:
I think the word “only” is misplaced there; I think a better way to phrase it would be:
It isn’t practical for a dictionary to list every sense of how a word can be used; the best dictionaries can do is provide a suite of general examples.
Your original sentence could probably be written either way; I would consider both grammatically correct:
However, the array of paint on a Pollock canvas is so widely varied, from an art perspective, it almost seems a shame to use the singular.