There are several hundred to choose from, in all different colours. And that's just B.davidii, without mentioning the other hundred species and their hybrids.
My picks (with max. 2M height and spread) would be Buzz Ivory for white. Sugar Plum is a fantastic colour, a deep cerise-magenta, as is Miss Ruby. Adonis Blue is a fine dark shade, but its growth habit can be lax. For Pale Blue, look out for Summerhouse Blue.
Regular and pruning in April will keep most, but not all, cultivars to about a 2M spread and height.
Anyway, my website (www.buddlejagarden.co.uk) has descriptions of many of the cultivars and species I have grown. Too many to choose from and too many to describe here.

You've not mentioned height but if you don't mind a taller one, my favourite is B. davidii Black Knight... or slightly shorter, White Profusion, though when the flowers start to fade, the browning is more noticeable than on the darker colours.

I have a smallish one with large fat purple pink flowers and the label had the name Pink. it was a proper label complete with picture so not just a hand written one. It gets about 6ftx6ft and I prune it down by 3/4 every spring.

Buddleia (and Hibiscus) are my favourite shrubs and have many of each ;)

For your requirements I would also suggest the 'Buzz' range, I bought 3 this year and have them all to stay living in pots.

My latest acquisition is Buddleia 'Wisteria' - as its name suggests it has a wisteria like habit :)

Sorry for the late reply. Many thanks to you all for your information and suggestions it’s much appreciated.

Bamboo - I bought a Black Knight this morning, managed to get it in the sales too £5.39!

Cor, well done - I couldn't get one anywhere a week ago when I needed one, had to make do with Buddleia Lochinch instead...