There's no colorless mana. The color of a person's mana is decided by two factors:
- Mana quantity decides the tone: less mana = lighter tone, more mana = deeper tone.
- Elemental affinity decides the hue: most living beings carry more than one type of mana, and the strongest type dictates its general hue (usually it's the color of their birth season, then secondly inherited from their parents' types). People with all affinities has pale white / rainbow mana, as when all colors are mixed together you'll get white light.
Devouring children are usually omni-elemental simply by the merit of having too little mana, there's no type significantly stronger than others (iirc it's mentioned somewhere by Ferdinand). But also because they have too little, their mana's tone is extremely light to the point of appearing colorless.
On the matter of mana signature: everyone, even feybeast or feyplant, have unique signature by their composition of mana. Rozemyne has her own too, but as a child with Devouring it's very malleable and easily dyed by other. It only took forever to revert back due to the Mark of Ewigeliebe (hardened mana clumps resulted from her escaping death's door multiple times), which acted like feystone and kept retaining Ferdinand's signature -- not to mention that she wore his protective charms all year round, literally being dyed 24/7.
Now, do gods consider it as problem? Probably not. They see thing in macro scale, as long as the country exist and flourish, it's not their concern whoever acquiring whatever schtappe -- good or bad. If a person has too little mana, they'll find their divine will at the entrance of the cave. But normally no such person get access to the Farthest Hall in the first place because noble society weeds out their rank before sending them to Royal Academy. Remember the knight who injured Rozemyne in Act 2 and got excuted? He was formerly a blue priest, but due to the country-wide mana shortage he was called back and still graduated normally. Which means the gods are very lax in handing out divine will, while noble society is more stringent with their rule.