metabloites
ignore the yellow water unless it becomes an issue.
if it IS an issue,
you have other issues that is way beyond yellow liquid.
ive noticed yellow goes with warmer temps,
and varies with strains/ species.
any good grow, your gonna see some yellow water.
gonzo CFL/ tube lighting will cuase yellowing also.
in that case, back off on light power, or switch to LED 450nm/ blue.
over long incubations will cause yellowing in some strains/ species.
in cultures on agar, yellow drops sometimes indicate hidden bacteria spots.