Hebrew Language Preposition אֵין allowed within סמיכות construct ?

[Jason]

I wasn't aware of the noun תְכוֹל for azure, only תָכֹל and maybe תְכֵלֶת . (For beginners rather confusing this diversity). It even seems that the ו in this noun is obligatory (mater lectionis); or does a color noun  תְכֹֹל exist in Hebrew ?

Assuming in the song we have תְכוֹל, then the תְכֹֹל given in the written lyrics I work with amounts to a spelling error.

Or is there another conclusion ?

I wouldn't take it as a spelling error. Modern Hebrew tends to write all o vowels with ḥolam. It's just the way of writing (especially in an unpointed text).