How dark is dark ?
And although the darkness of a fluid would have some bearing,,there are other factors, such as,,
1. Does the fluid look or smell burnt ?
2. Is the fluid contaminated ?
3. How long has it been since the last fluid change and what are the factory recommended service intervals ?
4. Has the vehicle been subjected to severe duty,,,such as racing or towing a vehicle ?
Power steering fluid should be clear in color when it is new,,if it is dark,,it probally is dirty and should be changed,,The cost of fluid is cheap compared to the replacement and installation cost of a power steering pump,,hoses,,and steering rack.
Transmission fluid is a dark red when it is new,,,and if it really is old and dirty,,i dont need to tell you how much a new transmission costs.