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eg. greenwidgets.com, bluewidgets.com
Through my previous observations and experiments, its quite clear that websites with keyword in domain easily rank well on lesser competitive keywords..but it adds up to my domain portfolio...and becomes difficult to manage large inventory of domains
what about having subdomains eg. green.widgets.com, blue.widgets.com
Just choose a good keywords domain, and added some relevant pages to the site. After that you can just leave it and see how it progress.
You can decide later on how you want the domain to function based on the SE traffic to your site.