Very, unless you have something of a known colour value close by, it's not easy to deduce the colour from a black and white or sepia print.
Want to try and guess at the colour of the insignia on these stug wraps?
You know the possible colours for the litzen and shoulder boards, you know the colour of the tunic.
I was shocked when I saw the pictures in sepia.
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"Truth is a shining goddess, always veiled, always distant, never wholly approachable, but worthy of all the devotion of which the human spirit is capable. "
Sepia printing was not in vogue in the period apparently. But I guess it could've been done.
Also some of the film stocks during the time were more sensitive to certain colours. making them very light and others very dark.
Of course if you've shot these with a digital camera and converted them with Photoshop there's no way of telling. 
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Indeed Mr Fibble, sepia was considered very old fashioned and only used as an 'effect' by club photographers. Some surviving period photos from the war years may look sepia now but that is down to poor processing destroying the silver over time.
Regular emulsions were generally orthochromatic and even panchromatic films of the time had limited red sensitivity - leading to 'swarthy' skin tones. Ortho films can be very misleading when interpreting colour.
As you say, mono conversions in Photoshop can change colour any which way, depending on channel control. If you have a known colour in shot you might have a chance at a good guess, but it would be no more than that.
