Node colour button has 3 blue dots and drop menu will give you choice which field to colour the nodes by.
For the colour key you should tick option under the Network view Tool menu >Style

If anyone wants to know the answer to this, I asked outside of the forum (support) as my colour key option was greyed out and node colour was already selected. Paola came back to me with the following answer;

If you wish to associate particular colours with values (e.g. Coca-Cola red, Pepsi always blue) you should go to Data>Manage fields> Options next to the target field>value order,colour,shapes

(this is available for Category type fields, but similar options are available for other data types)

Thanks Paola

Glad it worked! I forgot to mention, for those working with Network view, must see is our tutorial video:
http://tc.visokio.com/videos/?name=NetworkView&title=Network&lang=gb
(after 13 min you will know how to do everything!)