I think you should be able to do a JPQL or HQL along the lines.

SELECT count(p.colors) FROM Product AS p WHERE p.name = :name ... other search criteria etc 

or

SELECT count(c.products) FROM Color AS c WHERE c.name = :name .... other search criteria 

From Comment below, this should work:

Long colours=(Long) session.createQuery("select count(*) as cnt from Colour colour where colour.colourId in(select colours.colourId from Product product inner join product.colours colours where product.prodId=:prodId)").setParameter("prodId", prodId).uniqueResult();