As per JayC, that is part of its colour, some call it a 'go faster stripe' and it looks really great and I don't see anything to worry about. The shrimp that died may have been the increase in temperature from 66 to 78 as I imagine that happened quite quickly in such a small tank (although they would usually be ok at 78), so may have stressed that shrimp too much! Hopefully the other shrimp will be ok but keep an eye on them. Also as per JayC, the shrimp should be fine at 66 degrees.
Foodwise, you really shouldn't need much, maybe none, with so few shrimps, so if you are feeding bear that in mind, you should just probably feed very little every now and then AS A TREAT as there should be enough biofilm in the tank. You need to revisit this when the colony grows of course. Often shrimp will make a bee line for added food IF they are hungry but show little interest in added food if they aren't hungry as there is enough biofilm.
Welcome to the forum by the way!