CBennett: as the others have pointed out, most instruction sheets will give you a reference of the paints you need.
For example:
Tamiya - they only reference their own paints, which normally start with the letters X = gloss/semi-gloss or XF = flat colours. eg: XF-1 = flat black. Sometimes you'll also see TS = spray can colour. I think there are some conversion charts available that can convert the TS to an X or XF colour in their line.
Hence, when building a Tamiya kit (if you like Tamiya paints) simply follow their chart and reference numbers.
Hasegawa - normally reference both Gunze Sangyo and Mr Color paints. I think the norm is to use Gunze Sangyo, which all start with the letter H, eg H317 = grey.
If you are happy to use Gunze Sangyo and simply order colours based on the instructions/reference numbers, then it won't matter too much. It becomes more difficult if you want to use a different brand, and then converting the "H" colour to another.
These can be a bit trickier, esp if you're building a navy jet and it references H317 & H318 - which are both simply labled as "grey" by Gunze, however, i think one is light ghost grey, the other is dark ghost grey.
In those circumstances Gunze normally references the FS (ie Federal Standard) number as well - which (i think) is used by Model Master to differentiate between different colours.
So, this can also be handy.
Finally, it's quite good to locate a colour conversion chart - which i think is available on the net, which will convert colours from one brand to another. This can be invaluable to prevent stocking up on 4 brands of the same colour.
Hope this helps.