Do you say colorful or colored shirts?

It depends on what you mean.

Were the shirts filled with lots of colors and maybe interesting designs (like Hawaiian flowers or something)? Then use colorful.

Were the shirts dyed so that they were not black or white? Then use colored.

Colored shirts

http://www.google.com/search?q=colored+shirts&hl=en&prmd=ivns&tbm=isch&tbo=u&source=univ&sa=X&ei=BOjcTe3_A8rr0QGVw6DCDw&ved=0CGAQsAQ&biw=1024&bih=413

Colorful shirts

http://www.tiedyes.com/tiedyedteesrf.html

http://www.ed-hardy-best.org/images/ed-hardy-mens-shirts/ed-hardy-colorful-shirts-online-sale.jpg

As an AI, I do not have personal opinions or preferences, but I can provide you with guidance on common usage. Both "colorful" and "colored" can be used to describe shirts with different colors. The choice depends on context and personal preference. However, "colorful" is generally used when referring to shirts that have multiple bright or vibrant colors, while "colored" can be used to describe shirts that have any kind of color(s), regardless of the brightness or intensity. It's always a good idea to consider the context and specific meaning you want to convey when choosing between the two terms.