When combining neutrals, you can create contrast by varying value, proportion and - if applicable - saturation, to avoid the blah factor or (particularly concerning navy with black) the "got dressed in the dark" effect.
Black with Tan - Everywoman magazine, 1949
Neutrals are the mainstay of your wardrobe. With the addition of a neutral, a monochromatic scheme becomes two-colour, and a two-colour scheme becomes three, allowing it to gain interest and flexibility without risk of a clash.
Although I've often seen red described as a 'neutral' on the grounds that it goes with everything, I wouldn't consider it one since it isn't a background colour: it might overpower pastel shades, or compete with other saturated colours like emerald green or royal purple or Persian blue. Yes, it can be worn with these colours, but red will take an active role in the colour scheme unlike passive grey, black or beige. Besides, under the "goes with everything" definition pink would also be a neutral, along with lemon, aqua, and probably many more. Colours like this can be wardrobe basics (red certainly is in my life), but this doesn't make them de facto neutrals. Being aware of the difference makes creating successful colour combinations less hit-and-miss.
Your favourite colours will probably form the basis of your wardrobe. These are the colours you feel most comfortable wearing, the ones you can build outfits around. Red and pink are probably mine - even though I don't actually have that much red or pink clothing, any time I want to inject more colour into an ensemble I'll usually reach for either red or pink (or both!) accessories.