Fresh urine which has just been produced by the kidneys and has not yet left the body is sterile and has no smell, or only a very weak smell. This smell isn't usually very offensive but simply the particular earthy smell of wee.
But as soon as urine is exposed to air, when you pee it out, it starts to be broken down by bacteria which contaminate it from the outer part of the urinary tract and your skin. Then the salts in the urine, especially urea, are converted to chemicals such as ammonia which have a powerful smell which many people recoil at or describe as offensive.
If you're passing urine which already smells unpleasant then this may be a sign that there are bacteria in the urinary tract breaking down urine before it has been passed out. If this is so, then you may have an infection which needs treating.
The colour of urine varies enormously. The yellow colour comes from the excretion of breakdown products of bile, which is pigmented. Urine is usually very pale yellow or almost colourless if you're drinking a lot of fluid (because it then contains a large amount of water), and darker yellow if you're drinking less or even dehydrated, because there's much less water and a larger proportion of the pigmented bile products.
If you're jaundiced, with excessive bile pigment in your blood, then the urine will contain more of these pigments and look darker yellow, orange or brown. Blood in the urine can make it red, and rare diseases such as porphyria can also turn the urine red. But other things can colour urine, from beetroot in your diet (red) to asparagus (green and smelly!). Infection doesn't usually affect colour of urine. More important is whether the urine is cloudy - this may be a sign of infection although can be because of kidney stones too.
The explanation for strong smelling dark urine could simply be that you're not drinking enough fluid and your urine is very concentrated.