I think they have an iron or possibly a sulfur deficiency. First thing, make sure the pH is ~6.0 - 6.5 or the nutrients can lock out. next order of business, add a nutrient that contains iron (check the label). If that fails, spray with some epsom salts (~1 tablespoon per L).
Cannabis Symptom Checker - Identify Cannabis Leaf Problems (ilovegrowingmarijuana.com) Iron deficiency An iron deficiency will see your young leaves turning pale yellow and the buds will remain small and under-developed. Learn more about iron deficiency. Nitrogen deficiency Too little nitrogen will pale the leaves. Yellowing will occur between the leaves or they may turn completely yellow. The leaves start to sag and wither and old leaves begin falling off, Learn more about Nitrogen Deficiency