I had never given a thought as to which colour was "better" because I would think that one would be limitied mostly to the prevalent soil type in your area.
Looking into the BS3882:2007 standard I see there is no mention of colour, only of standard and suitability. (se more about this below....)
When buying topsoil, there can be a notion that is the darker, the better. Darkness in soils is often associated with an high proportion of organic matter but soils which are very dark and grayish may have been dredged from wetland areas. These soils can be very acidic and poorly structured and may not perform well in use.
Nevertheless, as a rile of thumb, quality soil is a rich dark brown or black colour. Of course, the colour can be affected by the degree of dryness of the product...... wet soil is always darker!!
I would think the likely call for the best soil would be "loamy," meaning it's made up nearly equal parts of clay, sand and silt.
To agreat extent the colour of soil is governed by prevailing local conditions..... our soil in our garden is a light sandy loam, it's good stuff and grows good plants, but some types of plants ofr crops will still need additivies and feeding to make the most of their potential.
Not so very far away from me here in Aberdeen the natural soil is a wonderfully bright terracotta clay colour... and it is rich in clay and sand and is extremely fertile though it looks quite different to anything you would ever buy .... looks aren't everything... it is content, drainage qualities, nutrient values and so on that are important. The BS rating lays out these areas.... but the real test is : does this soil grow good plants? If not, no matter what colour it is or how many labels someone has stuck on it or given it fancy names, it's duff!
One would hope that soil given a BS3882:2007 rating would be of good quality. It is up to the buyer to decide what grade of soil he requires
according to the use for which is intended - you'd need to study the recommendations to see what you needed for any particular project.
The RHS advice on topsoil says " Topsoil is generally available to buy in three different grades: premium, general-purpose grade and economy grade.
http://apps.rhs.org.uk/advicesearch/Profile.aspx?pid=375 to see more .
The five types of soil available from a supplier like Rolawn are Blended Loam ;
Vegetable and Fruit Topsoil; Beds and Borders Topsoil; Soil Improver; Organic Multi-Purpose Compost - which sound to me like a good way to make more sales!
This link is to a document that gives quite a lot of details about analysis of various topsoils :
http://www.envirolinknorthwest.co.uk/Envirolink/events0.nsf/0/8025739B003AADE3802574C80041DCCE/$file/080310%20Tim%20Evans%20-%20Soil%20Quality%20Assurance%20%20-%20BS3882%20-%202007.pdf