This is exactly why no market exists for its sale over the net - the weight of the product is great compared to its unit cost. Same with cement and other standard building materials. Its going to cost about �20, so you aren't going to save a bundle and the DIY sheds all have the paint colour mixing systems in-store.
I work for B&Q, if you keep your eye out there are always offers on, recentley we had 15% off all coloured emulsion and before that it was 3 for 2 on dulux emulsion. Also its 10% off for pensioners on a wednesday so if you have a relative over 60 could always ask them to go with you and save you a few quid.