This is more difficult than it should be, but I will give it a shot.
The Lavender color is the same as Self Blue in Old English and d\'Anvers. This color is also sometimes called Belgian Blue. Unlike Andalusian Blue, this color breeds true. So the offspring from a pair of Lavender chickens will all be lavender.
The color is a solid, pastel greyish blue. There should be no lacing. Males and females are the same color. This might be described as \'dove\' color.
The best description is a photo. Pictures can be found on the web at some of the Old English Bantam sites. Here is an example.
http://www.bantychicken.com/OEGBCA/articles/selfblue.htm