whats in a number

this is a question that has bugged me for a while now……all soviet and now russian military aircraft carry a large [buzz] number on there sides. red, blue, yellow, black. obviosly a ID of some sort. are these numbers and colours applied at random or do certain regiments have certain colours, or squadrons in regiments have different colours. apart from the national markings and number are there eny other squadron or regiment markings applied..what does that small marking found on some aircraft [ususaly under or near the cockpit] mean..