use carrirewave and mini-magick to change the png image's color

uploader.rb

class Uploader < CarrierWave::Uploader::Base
  version :full do
    process resize_to_fit: [960, 640]
    process :set_color
  end
  
  def set_color
    color = 'gold'

    # convert old.png -alpha off -fill gold -colorize 100% -alpha on new.png
    manipulate! do |img|
      img.combine_options do |cmd|
        cmd.alpha('off')
        cmd.fill(color)
        cmd.colorize('100%')
        cmd.alpha('on')
      end
      img = yield(img) if block_given?
      img
    end
  end
end