James, knowing how to predict the outcome of mixes mostly comes down to practice and experience with your own paints. Painting books and mixing guides, as well as guides you can find online and in posts on forums pF, are a useful starting point but when you get down to wanting specific shades and light-medium-dark for a uniform colour that's where you need to put the time in.
The very basic stuff like how to mix green and orange from scratch you probably already know, beyond that it's important to develop an understanding of what you need to mix to get bright or dull greens or oranges, depending on which you want. And then onwards to more subtle stuff like how to mix beige, olive drab, ivory, sky blue, that particular shade of khaki in a reference and so on.
The second part of your question - how colours work with each other - is right at the heart of the matter. First off remember you're mixing paints, not 'colours'. This can be very important because two paints for example can look nearly identical but actually mix quite differently, due to the pigment or pigments that they're made from.
Colour mixing is about paint, not colour in the abstract. This is why colour wheels and mixing guides can only be a starting point.
...
Need to correct a few things already posted:
technically the
subtractive primaries (the primaries of paint, inks etc.) are cyan, magenta and yellow;
the secondaries are therefore red, green and blue;
colour wheels show
visual complements only*, one has to look into mixing complements separately,
if one wants to use them.
*And only if the colour wheel is complete - must have cyan and magenta.
Some existing threads for more info:
http://www.planetfigure.com/forums/showthread.php?t=25756
http://www.planetfigure.com/forums/showthread.php?t=28808
http://www.planetfigure.com/forums/showthread.php?t=25163
http://www.planetfigure.com/forums/showthread.php?t=31372
Einion