Without a photo or mention of other symptoms, going by what you have said, it's most likely a water shortage issue, specially if its in a pot, but even in the ground, its been very, very dry. Leave the hose trickling at its base, there's nothing else to be done I'm afraid.

It could well be a watering issue, but could it be in the wrong soil and/or in the wrong position? Ceanothus like very well-drained soil and a sunny, sheltered site.

Thanks for your replies.Let me clarify,the ceanothus is in the ground,well watered as it is 10 ft from my garden tap and has thrived for the last 2 years,in fact this year grew in
abundence.However this last 2 weeks it started to rapidly change colour as the leaves lost their bright green glow and changed it's look.On checking the branches,the shrub still appears to be alive.I am hoping it's a temporary phase
and regrow the leaves later.I have lost other plants due to frost and cold but that was some years ago.

Plants will often grow for a couple of years in unsuitable conditions, and then start to suffer. As you've been watering it, it may well be a nutrient deficiency, but check out this link for information and other possible causes:

https://www.rhs.org.uk/advice/profile?pid=354