We can take a look at pictures and make a guess - but a diary would give us all the information we need to make a more accurate diagnosis... how old are they, are they autos or photos, what nutes are you using in what amounts, what is your pH level, what are your temps and humidity, is this an indoor or outdoor grow... pictures of yellow leaves are just pictures of yellow leaves... please, start a diary even if it's just for this one week - it will help US help YOU!
Thanks....