Nothing that cannot be mended.
You level your yellow mastery when you rise a level. You are presented with a screen where on the left are all your six masteries - with their level - and in the middle of the screen TWO card with two of the six colors. This means that for this time you can only master one of those two masteries. If one of the two cards is Yellow then ckick on it and you are done. If not, you’ll have to retry next level. You’ll be unlucky indeed if this takes more than 3 levels to comply.
The longer task you’ll face will be then to level your Kingdoms.
On the map every kingdom has a crest. On the crest there are stars and below a number.
By using gold you can level a Kingdom from 1 to 10, influencing masteries in the battles - Your mastery number stay the same and that’s what you are leveling now.
Once a Kingdom reach level 10 by what you will find be a lot of gold for what you gain at the moment, you will have a fixed bonus. According to the Kingdom it will affect Armour, Life, Attack or Magic. It’s usually a good idea to level Magic Kingdoms first.
By getting troops from that Kingdom, leveling them, adding traits to them and ascending them by sacrificing copies, you add stars to your Kingdom.
An active guild will give you, in exchange for your gold: souls, keys for all kind of chests and diamonds for the Soulforge. Other bonus will come from fighting in Guild Wars.
Good Guilds are usually lenient toward new members leveling their Kingdoms.
For exemple, my Guild has a rather low requirements of 100.000 gold, 50 trophies, 500 seals, but GW mandatory. Other Guilds might have a completely different approach. We accept new members Active Players, rank 178.