The letters in the codes refer to different groups of genes.
The important groups are:
A, B, C ,D and P
Each mouse has two genes in each group, one from each parent.
Wild mice are agoutis and they have the code AA BB CC DD PP which is the basic genetic make up of all mice. So in order to make the mouse a different colour you need to introduce a different gene.
So with the A group
A is agouti
a is black
because the agouti gene is shown as a capital 'A' it means it is a dominant gene. This means that if the mouse has got an agouti parent and a black parent it will have the code 'Aa' so although it has one black gene the agouti gene is stronger and the mouse will be an agouti. To be a black it would have to have a little 'a' from both parents and its code would be 'aa'.
Other colours which we commonly see are caused by dilution genes, these are all recessive means they are like the black gene 'a' and need to get one of each from both parents.
The B group controls whether the mouse is chocolate.
chocolate is 'b' so a chocolate mouse needs to have a 'b' from each parent. The 'B' gene has no effect on colour so if a mouse is 'BB' its colour will be decided by other genes. If it is 'bb' then it will be chocolate.
The C group is very complicated and has various genes which can make a mouse albino, cream, silver and others.
The D group talks about mice being Blue and works in exactly the same way as the chocolate gene.
The P group controls eye colour.
'P' is black eyes
'p' is pink eyes.
If a mouse has pink eyes it sometimes has the effect of making the fur colour lighter too. So two mice can have exactly the same code until it gets to the P group and they can look entirely different. For example
aa BB CC DD PP-is a black
aa BB CC DD pp-is a dove
Another gene that is very important is 't' for tan. It has a little 't' but its actually dominant so the mouse only needs to have one tan gene from either parent and it will have a tan on the belly.
Other genes particularly those genes that cause markings work in their own ways and I find it easiest to look these up as and when you need them, theres no point learning all of the genes as you might never have a mouse which has these genes.
I hope that is a little bit clear, I just read over it and Im not sure that it is. Sorry
What genes in particular are you interested in. And what mice are you breeding/aiming to produce?