For me, wearing a "letters sweatshirt" other than in my sorority's colors is about expressing uniqueness as a person, and simultaneously announcing my affiliation with pride.  (My organization doesn't mandate that letters be in our colors only.)
I'm really excited about a letters sweatshirt that I'm having made: green sweatshirt; the foreground material comprising the letters are a shamrock print (green background, darker green shamrocks outlined in gold).  The background cloth will be black.  The embroidery stitch will be a dull, yellowish gold.  
I chose this combination because it represents the grand tradition that OU's College of Engineering has - it incorporates color (green, black and gold) and symbol (shamrock) that represents the College.   

  If I really went all out, I'd be looking for print cloth with cannons and green owls, which are also big symbols of the tradition.
I actually thought about getting letters in royal blue and silver (ASK's colors).  Since I wear a lot of clothing already with combinations of blue,silver, sometimes white roses (often with a lavalier or member pin), I chose this as a way to represent two affiliations that I'm proud of simultaneously, rather than just one.   

  There's no way anyone could mistake me for anything other than an ASK and an engineer, with that set of letters!
------------------
equeen
A Lioness has her Pride!
@>--;--
Alpha Sigma Kappa - Women in Technical Studies