stems are purple and leaves are yellow

let me know what i should do

It might be a nitrogen deficiency if the chlorosis is occurring on the lower fan leaves.

But you should really tell us more. How old is the plant, what is it growing in (medium type/brand), what is it being fed if anything, and what is your general setup? Also what the problem is exactly; how long it has been going on, where it is happening.

Pictures also really help.

yeah pics and info would be helpful. We also dont know if its overwatered or not. If its not overwatered just add some nutes. If your plant is small be sure not to overdo it.

if you have; stunted growth, smaller leaves, leaves turning bluish/green(yellow eventually),blotches in advanced cases, and leaf steams(petioles) turning purple starting on the underside is a lack of Phosphorus. From medical marijuana growers bible. Progression of P deficiency at a glance: stunted and very slow-growth plants, dark bluish-green(yellow) leaves, often with dark blotches,smaller plants overall, when blotches overcome the leaf stem, the leaf turns bronzish-purple, contorts, and drops. If all you leaves are yellow could just be lack of nitrogen, plants eat that up in veg and still need some in flowering. Though note, some strains naturally have purple petioles.