Why did Bart toys have a blue t-shirt when in the TV series he always wears a red t-shirt? Did he originally wear a blue t-shirt and have it changed to a red t-shirt at some stage?
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: