Mild Discolouration

if it never progresses (gets worse) i'd chalk it up to genetics. Do check pH and make sure substrate is somewhere in safe range. 5.8-6.8 -- slightly acidic levels approaching top of this range is best for soil. soilless/hydro i'd go with 6.0. usually this clears up when i see it on lower/older growth. If it continues above, but again unlike deficiency or a toxicity it's just how the leaves are grown and not changing size or scope, it's fine as long as new growth does not display it. In 3+ years i've seen one where it did not abate. The plant had some sort of defect that impacted "whatever" process etc etc. if it changes shapes ore spreads, then you can consider environment and nutes. definitely need more info for that...