https://www.google.de/search?q=select+plastic+bottles+by+colour&ie=utf-8&oe=utf-8&gws_rd=cr&ei=8PJ-VpaJC4H5UpukukA

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Plastic-bottle sorter discerns variety of colors

January 1, 2002

Vision-based sorting system helps recyclers distinguish a spectrum of plastic-bottle colors using custom image-processing software.

By Jeff Child, Contributing Editor

In the past, most PET (polyethylene terephthalate) plastic beverage and water bottles were either clear or green. Bottle recyclers sorted these two colors with vision-based systems. Recently, an array of colored bottles has entered the market, a trend that started in Europe when light-blue tints were added to clear bottles of water. That expanded to more colors as vendors sought to tie distinctive colors to each of their product brands.

hope this is helpfull for you

Peter

I will not tell how to make because external components as USB camera, special light and a lot of code lines is needed. Instead of starting building from null better use well known systems, eg. as added pdf. Of course, there are many of them - depending size, IP class, resolution, price. But of course if You want, you can take into consideration Arduino TCS 230 or/and TCS3200D. And again - it is kind of DIY system, but if You do not want to write code always electronics such as 'light to  frequency' converters may need your requirements: https://www.sparkfun.com/datasheets/Sensors/Imaging/TSL235R-LF.pdf which is working perfectly with DiscoVery 32 (STM32Discovery). Systems of  DiscoVery and/or Arduino should  cost less than 200 EUR.