I don't know if I understood well , but my guess is you want to host both blue22.co.uk and cola.com on DH and you want to access cola.com either by typing cola.com or by accessing blue22.co.uk/cola , right ?
If that's the case then you should fully host blue22.co.uk in you cpanel , let's say in webdirectory : /home/username/blue22 . Then you should fully host cola.com in webdirectory /home/username/blue22/cola .. that way cola will be a subdirectory of blue22.co.uk , but will be also accessible under cola.com
As for the email , it depends on your registrar ( where you log to control your domain name ) but most of them will just let you change the dns servers to point to any dns , in this case :
This mainly means that anyone that will somehow try to use your domain name either in web or email will be redirected to DH servers .
After you fully host both your domains as I indicated before you should update your registrar data to be the same as above , then you can go to your DH panel and add user emails , you can then access those by typing : webmail.cola.com for cola emails or webmail.blue22.co.uk for blue22 emails .
Hope that helps .
Regards , Manar