If you're planning to install a resistor in place of the thermistor (these are very different components) I don't recommend it.
(First Rule of repairs: DO NO HARM.)
I have disassembled the pump from the manifold, and removed the impeller housing, isolating the pump motor. I started writing a disassembly procedure along with editing the seventy pictures I took for inclusion. I'm still checking out the way the pump body is attached to the motor shaft (could be crimped in the middle) to try to open it up without destructively cutting the casing. There's a circuit board at the rear of the motor where the thermistor lives along with some other components. DC motor operation is pretty straightforward, but I need to figure out what components are inside, so if I have to cut it apart to do that, and verify how the assembly is sealed, I will. The long-term goal is to create a schematic, and build a 3D model to create a drawing of the seals, O-rings, bearing race, etc. for anyone who wants to rebuild their assembly at some point.
As for the wire color, mine are grey and tan as well.
When you measure the resistance across the thermistor, what are you getting?
CC