leaves getting blotchy dark spots

im new to growing and one of my plants is getting dark blotchy spots on the inner part of the leaves immediately after the leaves stem ... this is only on 2 of the leaves coming off of the same node these arnt new leaves but some of the older ones new growth seems to be fine also not the main stem but the ones holding the leaves are a dark purple this is all bag from different plants ive heard that cold could create the purple alot of the plants in the same conditions dont have the purple stems anyone know if I should do something to better the leaves or plant any help is appreciated

Could be nutrient burn could be the coulour of a strain you are growing comeing through plants often go a purple coulour if the temps get to cold.Could be any of the above.