How to tell which way a colour is leaning?
If you are looking to identify how a color is mixed first and foremost you need a color wheel which will show you where the color you want lands relative to the primaries. Second, read up on color theory which will help you understand what is going on with that color wheel and how colors behave in general.

Once you have that down, here are some tips for keeping your mixed colors vibrant (or not):

First... for any two colors you are mixing, be sure their value/lightness/tone are similar. Use white to bring up the darker color to match the lightness of the lighter color.

Second... there are cool and warm versions of the primaries (Red, Yellow, Blue). Cool colors tend towards blue, warm toward red. If you mix a cool color with a warm color it will lose intensity, so mix cool with cool and warm with warm.

If you keep that in mind your mixtures will be cleaner. As far as which colors produce which, look for a color wheel. But you probably know the basic idea: Primaries lead to secondaries: Red+Yellow = Orange, Red+Blue = Purple/Violet, Yellow+Blue = Green. A secondary color mixed with its "opposite" (the primary -not- used to form it) will desaturate the color, eventually leading to a chromatic grey.

You should be able to get most any color you want using warm and cold primaries and white. However if you know the secondary color you want to use start with that as a base pigment and "customize it" using small amounts of other colors. That is where experimentation and experience come it.

Finally the concept of relative color comes in. That is how one color is affected by the colors adjacent to it. A color next to its complement will appear more intense as the contrast is greater. Look up Joseph Albers who did extensive research into this. 

The paint binder (acrylic, oil, ...) and scale of the work do not effect the color theory, however cheaper paints will have less pure pigments and therefore will have less intensity and will not handle mixing as well as professional grade paints.