Pink in the seams or the plastron doesnt always indicate to septicemia,my turtles had the same thing a month or two ago.
My vet gave my turtles some betadine solution and silver sulfadizine cream (i think thats what its called) and an injection and told me to add more variety to theyre diet and to stay away from treats like shrimp,the pink is still they're but my vet told me it can stay they're for quite a while.If he has been acting normal for two months that he has had the pink on the plastron then its probably dietary.
Also can you please fill this out

:
How big is your turtle?
How long have you had it?
What is the water temperature?
Are you using a water heater?
How much water is in there?
Are you using a water conditioner?
Are you using any filtration?
What is the basking temperature?
Is there a basking light?
Is there a basking platform that is easy to climb on?
What kind is it or what is it made out of?
Is there a UVB light?
What have you been trying to feed it?
When was the last time your turtle ate?
How big is the tank/pond/enclosure?
Is the tank near a window?
Is the tank in a room with a lot of activity?
Have you read the Basic Care section?
Have you searched the forums for similar situations?
Is there any other unusual activity/symptoms?
Anyways here is a few links to how a turtle with septicemia looks like:
http://www.austinsturtlepage.com/Care/m ... Septicemia
http://www.redearslider.com/forum/viewt ... septicemia
Here is a few links to similar posts like yours:
http://www.redearslider.com/forum/viewt ... k+plastron
http://www.redearslider.com/forum/viewt ... pots+shell
http://www.redearslider.com/forum/viewt ... k+plastron
P.S. just because nothing serious was wrong with my turtles doesnt mean the same for yours,i would still have him checked out by a vet.