On the web the colour look good and dynamic because of monitor which uses RGB as primary colour for light. You can easily fooled by the colour from monitor. Cosmetic colour are make of pigment and solvent which will look different in real situation. Anyway there about 17 millions combination of colours. To get the exact colour is more difficult than winning ToTo.

There is definitely a slight difference. It also depends on the quality of the makeup. For example, if you buy in stores, the colour of the makeup you see and when you apply on yourself is different. Depends on intensity of the colour pigments. ie some makeup brands have greater intensity than others so the colour is more intense.