Are they in enough light? Is it just the new leaves or are the old ones also changing colour? You will need to water them regularly until they get their roots down to some moisture. I assume that they are in the ground? What sort of climate do you have?

thanks for the response
only the old leaves are changing color. they get morning sun and couple of hours in the afternoon. so regularly will be everyday for the next two weeks? yes they are in the ground. we are in late spring early summer here in perth western australia

I wouldn't worry then if it is just the old leaves. Remember, trees remove the goodness from their old leaves in autumn and change colour because of this. Your plants are probably doing the same with their old leaves as they don't have an autumn! Don't forget to give them an occasional feed to keep the colour going.

YOu don't say how big the plants are - do they yet have a "trunk" or woody stem, or are they smaller, no woody stem yet? If you bought them with the woody stems already, it's possible the lower leaves are getting ready to drop and fall, which is why they're changing colour. Some lower leaf loss is normal, but depends how many are doing it. Just keep them well watered - in Britain in summer, that would mean a good soak every 4 or 5 days (when the temperature is about 70-75 deg. F, for comparison with your temperatures), and by a good soak, I mean probably a sprinkler for 20 minutes minimum. Two weeks of this won't be long enough, keep it up for up to 3 months, as you're in Spring, unless you get lots of rain.