In time this will become easier and easier. It sometimes helps to squint at a color to see what it would like in black and white. If you take a photo of the color you are painting with and convert it to black and white, you will be able to see its value. It will be some shade of grey. This grey will fall somewhere between 1-10 on the value scale. Every color has a particular value. However, the value of a color can be accentuated by what is next to it. For example, if you have a pale, yellow color, this will most likely be a light value; maybe a number 3 on the value scale. If you would like to make it seem lighter, place next to it a color that has a value scale reading of 9 or 10, like dark purple. The dark value color will make the light value color seem lighter.