# controller
@image = Magick::Image.read(@design.photo.path).first
average_color = # your magick method to return the average color
# so average is like " {:r=>155, :g=>132, :b=>118} "
@average_color_string = "##{average_color.values.map{|v| v.to_s(16) }.join}"
#view
Average color: <%= @average_color_string %>
should display something like this:
Average color: #9b8476
The .to_s(16)
converts the Integer into a 16-based string, also known as Hexadecimal.