As Brian said, the FS is the system used on full-scaled items, it is used to differentiate slight colour varitations.
The first number indicates sheen level (1 = gloss, 2 = semi-gloss, 3 = matt)
The second number indicates what general colour it is (0 = brown, 1 = red, 2 = orange, 3 = yellow, 4 = green, 5 = blue, 6 = grey, 7 = other (white, black, violet, metallic), 8 = fluorescent)
The last three numbers indicate intensity, the lower the value the darker the colour.
There are a couple of online conversions, the best of the lot would be Urban Fredriksson's Color Reference Charts,
Part One groups paints by country and time period,
Part Two groups paints by manufacturer. So if you know the Humbrol number, you can find what the equivalent Tamiya paint is etc.
Specifically, if you go
here there is a summery of all that information, all you need to do is look for the FS number and read what paints match it.
Cheers,
Linz