Another thought -- it may well be possible to configure the ATTiny to consume so little power that it can sit in standby and still have a battery life that is dominated by the self-discharge of the battery. In that case, a solution consisting of just an attiny, 3 LEDs, and 3 resistors for those LEDs would work and is as minimal as possible.
I made my last message more confusing than necessary by placing the FET so that it broke ground instead of VCC. Sorry about that. Here's the same circuit, but flipped upside down so that it's VCC that's switched, and I've included the other FET that protects the ADC input. This circuit works, I believe.
The line under test and R2 form a voltage divider, the voltage at the central node there is determined by the resistance of the line.
M3 is a P-channel FET, which turns the rest of the circuit on when a line is connected.
M2 is an N-channel FET that connects the ADC to the voltage divider when M3 is on. Without it, current would always be flowing through R2, D1, and R1, and the device would never turn off.
C1 is the capacitor that should generally be on an ADC input.
Ignore R4, I was just using it to measure current. D1, D2 and R1 are just parts I used to simulate an attiny and LEDs, they're not part of the design. (As a side-effect of the way that ICs are manufactured, there are often parasitic diodes running from every pin to VCC, and from ground to every pin. If the voltage on any pin exits the range from VCC to ground, these diodes start conducting, which is bad news.) The values of C1 and R2 are only approximate and you should design them properly. Also ignore the part numbers there for M2 and M3, those parts are probably reasonable choices, but almost any low-gate-threshold voltage FETs will do.
I know you were only asking for hints, but it was an interesting problem, so I got a bit carried away -- thanks
! My solution is just one possible approach, there might be a more elegant way of doing it. Using a dual opamp (e.g. AZV832) instead of an attiny would remove the need to program the device, and make the process of calibrating the cutoffs for the different colours a matter of turning trimpots instead of reprogramming. Pros and cons there.