Here, dei is a partitive article. Read more on partitive articles: http://www.italian-online.de/grammar/chapter6/6_9_4_partitive_article.htm
Could some body tell me which is the rule to decline or nor the colors. I have seen that some of them could be declined accordingly to the gender-number of the noun that is describe e.g. bicchiere rosso, bicchiei rossi, but other colors like arancioni or verde has not been declined.