To re-iterate much of what is above, there's 3 basic painting methods..Opaque only, transparent overlay and a mixture of both..
Opaque only-Opaques are much easier to control as they will only get to a certain color or tone and generally won't look patchy if you take the color to its full intensity. They cover the layer before so mistakes are much easier to fix, main problem with opaques is they look chalky due to the white that's mixed in, Its hard to create anything realistic looking with an opaque only system..
Trans Overlay- This is primarily using transparent paints only, sometimes it is done on top of a white layer if your using a dark background. Here you want to use the white of your canvas as your white and try to avoid covering it all..As the trans is transparent, the colors look brighter than likely what you see in your mixing cup, they can look much more lifelike and realistic when done correctly but are much harder to control and learn the ins and outs off and you have to avoid coloring over a previous layer unless your using transparents of similar tones..IE Add Blue close to or over a yellow and you'll get a green tint
Both systems-Likely the choice of most and a system that has been utilized for eons in oil and acrylic painting with final glazes/washes being painted over a opaque base..The trans being painted on top of an opaque allows a huge range of color possibilties and helps remove the chalkiness and gives nicer shading to an opaque only painting with can look flat and 2d without that help from the transparents...
Ultimately I would suggest that most beginners become comfortable with opaques first as mistakes and such can be easily covered up, when comfortable try finishing your opaque paintings with a transparent color or two to really pop your pics and add depth..If you want to try transparent overlay, a very good understanding of colour and how it works is critical as much of the mixing takes place on the canvas..Though there are many reasons to break the rules, even if just for fun as it will teach you the do's and don'ts pretty quickly that will suit your own future desires..But taking the pain out of it to begin with will provide most with much more encouraging results...
Traditionally in most painting styles black and sometimes pure white is avoided at all costs, personally I luv to use black as many do as it provides some great contrast, especially in the darker subject matter that most AB artists take on, but in regards to portraits and landscapes and other traditional area's try to take black out of the equation all together to get more realistic works as even the darkest shadows, generally have color to them..GL