Maybe a dark grey for the decking and fence and a different colour for the shed? Some cheap ideas for adding colour and interest: remove a patch of grass and add wild flower seeds. Attach trellis and grow a few climbers - perennials like honeysuckle or clematis or annuals like morning glory. Add some hanging pots to the fence and plant nasturtiums seeds, dig some borders and add perennials whilst they're small - cheaper at the start of the season. Add pots to the decking and grow herbs or flowers.