Ok trying not to get scientific
1st image is the colour spectrum which we should all be aware of.
2nd the colour/grey wheel gives a indication of how colours change once they are changed to mono probably not truly accurate in this example
3rd image is a greyscale, the human eye can see 31 shades of grey, if you're having problems seeing the 7 shades here from white to black you have problems.
So what does this mean, when shooting in mono you have to be aware of the way colours relate to each other, study the grey wheel and you can see that different colours have a similar grey value. All is not lost however, to turn your image from an effectively neutral grey into a more punchy picture, do the mono conversion in your chosen software, you should have with that tool, a panel similar to the colour wheel, by adjusting each colour value this can change the mood of your picture, give it go.
Don't use the "Contrast Slider" by manipulating the individual colours, you get a more professional look. don't forget to check shadows and highlights