[Amps] difference between 2,3,4 and 43, 45, ... materials?

[Dan Sawyer]

All,

I am constructing a bi-directional coupler according to the notes in the 
arrl antenna handbook. It calls for a couple of transformers to be 
build. I have purchased Amadon 'red' cores. (I think perhaps Yellow 
would have been a better choice.)

The local electronics store seems to have two different types of core 
materials, one based on colors and a 2,3,4... series covering a 
frequency range. The other has series numbers 43, 45, etc.

What are these two series used for?

What is the difference?

Within a frequency range do they functionally similarly?

Thanks in advance - Dan