Yellow leaves are usually a sign of stress and without more information I can only generalise.
Could be either over or underwatering.
Could be you've just transplanted and the plants are stressed from that - they'll recover.
If the plants are established, have you left enough space for each parsley plant?
Could be not enough light - maybe overcrowding as above?
Whichever it is, pick the leaves off and don't use them. The next stage from yellow is brown and dead.
If you adjust your watering, and anything else that may be wrong (from the list above) then the number of yellow leaves should decline and the parsley plant grow healthy again.
Keep cutting the healthy green leaves from the plant.
