yes, according to the guides they're available only in a premium locked field
without it I would have given up. Yellow, Red and White are necessary for high-level play. You can play and even beat this game without them, but you'd have a much harder time at it, and need to grind for an extra 6 hours or so just to get the requisite resources, but it can be done. If you're only playing this for the story, the pouch is not necessary.
I agree, not having purchased any premium content, that's exactly why i gave up playing that game.
Basically the game difficulty is linked to your starting mana in most fields.
Not enough starting mana and you'll have weaker gems or less ones deployed from start and from there it's a race in which you're always trying to catch up until the last part in which you don't deliver enough damage to beat everything thrown at you.
Enough starting mana and you start very well, and keep up with the monster power progression.
Now if you increase your skills too much, you lose a lot of that starting mana that depends a lot on your unassigned skill point (that's in my opinion is bad design, a game in which you need to keep your upgrades low in order to win on some fields ... but whatever)
A workaround to keep skills not too low while having enough starting mana is then to level up ... level up a very lot (as the talisman fragments that give +mana does not give much anyways unless you're insanely lucky in finding good ones)
To level up a lot, you will need to replay fields with traits enabled, and make those traits as high as possible.
And that's the problem : increasing field traits will cost a lots of core shadow (more than 130 at the point i decided to give up), and when you're finished playing such a field, you will have gained between 2 or 3 levels yes, but you will have gained only between 10 and 20 core shadows most of the time !
Meaning you will have to go grind again and again to get back enough core shadow in order to replay fields with highest traits power (to get the best XP possible).
And that without mentionning that fragments will cost lots of core shadows too.
The fact that the gems (and gem skills) that makes the best combinations according to every guides are premium locked, meaning too that you will be stuck a lot in many fields as you actually available gems will not be as powerful as they would be if you had the premium locked ones+corresponding leveled skills.
Meaning that you'll need more starting mana to counterbalance the fact you'll have weaker gem and keep up with later field difficulty, meaning you'll have to level up again a lot more.
Back at the core shadows grinding due to expensive traits, and repeat ad nauseum.
To me for non premium play, Chasing Shadows is the worst game of the serie, even the many premium locked content didn't prevented the player to have fun in Labyrinth in which it was better balanced between premium and non premium, but in Chasing Shadow it forces non premium to grind a lot more than in Labyrinth.
And despite i usually have patience, it was frankly too boring at that point in Chasing Shadow to grind again and again.
Definitively a version of gemcraft in which you want to get the premium content to cut the very boring grinding and enjoy it.
The good side for me is that giving up on chasing shadows unlocked more time to play something else