"Liquid food with iron for plants with yellow leaves" translates as "Alimentos líquidos con hierro para las plantas con hojas amarillas" in Google Translate (useful tool).
If you go into Sol D'Ebre to the counter at the back of the big shed at the rear and present a piece of paper with the above written on, you should find that one of their resident experts will be able to help you.  It may help to know what type of plants are affected by the yellowing.

Why do you think you need Iron ?

Plants main food is N,P,K.. so all fertilisers are graded by this ratio, eg. 8:2:2 [high N for growth] or  1:3:10 [for flower/fruiting]

Most people kill pot plants by overwatering, rotting the roots, overfeeding is also very common and easy to do.

Nitrogen deficiency, if you have it, is usually shown as a yellowing, from the bottom of the plant.