As you probably know the plastic material Lego is made off hates direct sunlight or too bright lamplight, as almost every plastic material does. The parts will lose color, especially the colors white, blue and light gray. So keep them away from there. The same applies to your boxes and building instructions. They will bleach under too much light. The same thing happens if you keep your models or parts in normal day or lamplight for a long period (several years). So if you have a box with Lego parts to play with, put it in a dark place if you don't use it for a while. And if you expose your Lego model for a long period in your room (possibly in a glass case to prevent them from becoming dusty), it will also inevitably lead to discoloring.
Dust and dirt
Playing with you Lego will inevitably lead to dust and dirt on your models or parts. That's no problem for the plastic material can easily be cleaned (see below). That is a comforting thought, especially when you want to give your models or parts away or when you want to sell your Lego.
Cleaning your Lego in a dishwasher is a bad idea. The temperature is too high, the detergent is too aggressive and the parts will fly around in your dishwasher. Although some people use a pillowcase to prevent the last, it is still not recommended. Cleaning your Lego can best be done in a bath with lukewarm water and a detergent without bleach. Leave it there for a couple of hours (you may carefully stir now and then) and rinse off with cold water. You can use a soft brush (for instance a painting brush) to remove dirt that hasn't fully dissolved. Shake dry and let it dry in the air on for instance a tea towel . Don't dry it in the drier (also not on a low temperature; it will always give scratches) or in the sun (read above). You can use a hairdrier on low temperature if you or your children can't wait to build. Don't forget to turn pieces with hollow parts on both sides after a while.