The key thing to remember with African cichlids is that they need clean water. It is so important that you clean the tank using a gravel vacuum at least once a week. If they don't get clean water, it can lead to diseases like popeye which can be hard to treat if you don't maintain good water quality. The water test kit is your best friend with these guys.
Another key thing to remember with Africans is that they are very territorial. They are the kind of fish that you have to overcrowd to reduce aggression over territory. Yellow labs are some of the less aggressive Africans, but they are still Africans and can't help but be territorial. Provide lots of caves, plants, rocks, and other hiding places so that the subordinate fish have some place to hide from the dominant fish.
I'm not sure how large 40-50 litres is, but I recommend no smaller than a 30 gallon tank for more than one African. Also, if you only have 2, then the dominant fish will pick on the other until it is dead (that's how Africans are). The recommended ratio is 1 male to 3-4 females. That would require (in my opinion) at least 30 gallons of tank.
There's a lot of research that goes into this, but once that's done the fun can begin. Just make sure you do your research on the fish you are interested in before you buy them. I guantee that your fish keeping experience will be better if you do the research in the beginning.
Good luck. Looks like you're off to a good start!