Why did Bart Simpson toys have him wearing a blue t-shirt?

I have found no definitive reason why this is the case, only a series of possible answers to this. It would be great to find an interview with Matt Groening with an answer to this.

First of all, early pictures of the Simpsons family from the days of the Tracey Ullman Show do sometimes have Bart in a blue shirt.

(Low resolution picture from Wikipedia - uploaded to Wikipedia as 'fair use')

However I have also seen on YouTube the early character with an orange colored shirt.

There has been some speculation that the blue shirt is an easy way for some counterfeit merchandise made from pictures of the show to be spotted, as it will typically have a red shirt.

The blue shirt for merchandise, even if it started accidentally has become a 'meta joke' within the series, witness the episode Pokey Mom from 2001:

Homer:  That's not going to do it, Marge; you need something red.  
        [picks up Lisa, and waves her over the stadium railing]
        Hey, Toro!  Here's something to gore!
 Lisa:  Da-a-ad!
Homer:  Not now, honey, Daddy's busy.
        [it works; the bull begins to charge toward Homer and 
        Lisa]
        [pulls Lisa back into the stadium] Now, for a little 
        calming blue.  [turns to Bart] Hey, where's your blue 
        shirt.
 Bart:  I don't have a blue shirt.