Even though many people consider these terms to be synonyms, the case is a bit more complicated: though wholemeal bread is dark, dark bread doesn't have to mean it's wholemeal. What is more, it has often happened that the bakeries have dyed the bread with additives forbidden by the Polish law (ammonia caramel and sulphite caramel)
You should definitely read:Is bread really fattening?
The ability to distinguish wholemeal from dyed bread is a precious one, not only because of the different nutritional value of the products, but also the presence of unwanted ingredients. How to not be misled, then?
The simplest way is to read the label, of course. If the first ingredient is "wheat flour, type 500", then you don't have to bother reading on. And slogans such as "multigrain" don't change a thing, because sunflower and pumpkin seeds among the ingredients do not mean that the bread has been made from wholemeal flour. Wholemeal bread has wholemeal flour mentioned on the label, period (wholemeal
It's worth to take a good look. Wholemeal products are never fluffy and high-risen. They usually are quite dense, and if you take a close look you'll see they're made of coarser flour, and most importantly - they will be heavier than a regular white bread of similar size. I do realize that overtouching and ogling a loaf in a supermarket is not particularly elegant, but you only need a little bit of practice to do that in a hygienic and discreet way.
The final method of verification is "the leap of faith". You can simply ask the cashier if the given bread is wholemeal or only dyed. Most grocery store employees know very well - although it might seem otherwise - what they're selling. What's more, you only need to buy high quality wholemeal bread once and have a good look at it at home to be able to tell it apart in the future.
Conclusion: wholemeal bread and dark bread are two, very different, categories of food products. All you need is a little bit of attention while reading the label or a good look to be able to tell the difference.