Re: Lightening acrylics
Acrylics dry darker - yes this is normal. The dried final color is the true color of that paint - the color to keep in mind. I like to paint a thick swatch on the tube near the mouth - that way you can see its final color, and when you open the cap you can clearly see the wet vs. dry difference. Oils have the luxury of no color change (although oils tend to get less glossy over time).
The better acrylics have less value shift, although there will always be some. It's best to stay with artist-grade/pro acrylics if you want to minimize it. The darker and more transparent colors experience more value shift. Minimizing the water you add will also reduce it. I like to use acrylic medium rather than water - the clearer the medium (less milky) the better. Also, rather than adding more water or medium to help spread the paint, you can just squeeze out more paint...the more paint you use, the longer it stays wet and workable.
For your color problem of getting the correct green - I would usually do it the way you mention (adding a bit of white and yellow) ...but yeah if you over-do it you will get a green that is too light, too blueish/chalky or too yellowish. With practice you get better at pre-adjusting to just the right amount. The best way to pre-lighten a red or orange might also be by adding a bit of white and yellow. For blues and purples, just a bit of white usually works. But it depends on the circumstance - you have to try and see what works best in a given situation.
Color is the most important element in painting - except for everything else