courgettes

Where are they at the moment?

Soil or compost?

Indoors or outdoors?

How long have they been growing?

Yellow leaves can be a watering problem, nutrients, or cold.

Unlikely to be nutrient if you've only recently potted them on.  Unlikely to be cold if you've only just put them out.  Have you been overwatering?

If watering is fine, you might try a general purpose fertiliser, either seaweed extract or similar.