What is happening?

Looks like a copper deficiency, A cannabis copper deficiency appears with leaf symptoms such as dark leaves that take on blue or even purple undertones. The tips and edges of leaves turn pale yellow or white in stark contrast to the rest of the leaves which have turned dark. In flowering it’s important to correct a cannabis copper deficiency as soon as possible because buds may stop maturing if the plant isn’t fixed up right away. Copper doesn’t move easily through the plant and is considered “low-mobile” which means the yellowing leaves might not necessarily turn green again, but the problem should stop spreading to new marijuana leaves. The most common reason growers will see copper marijuana deficiencies is when the pH at the roots is not in the correct range. Copper tends to get locked at certain pH levels, and is better absorbed by the plant in a slightly acidic root environment. Hope that helps and good luck to you