If I had a magic wand for beginning artists, I would give them the gift of drawing and colour theory. Both are areas that I see people struggle with and there is often an unwillingness to invest time in what are the backbones of art. Without being able to create an underlying structure freehand and without knowing how to create colours and values, people become frustrated and unable to achieve their artistic potential.
Artists need to understand colour theory as much as they need to understand the importance of values. These two areas are often a weakness where timidity and lack of technical skills play a part. Both have a solution that involves some study of theory and some time spent practicing to really understand the rational behind the process. I promise there will be an "Aha!" moment when it falls into place and opens the door for you to art without as much struggle involved.
Greys and browns
The important things to remember are hue, which is the colour pigment itself; value, which is how light or dark a hue is; and chroma, which is the saturation level of a hue. Comparison of a colour to the range on a grey scale will provide you with an idea of value. If you turn your colour or painting into a greyscale image, you can easily do the comparison to find where it sits on a value scale or compare it to a reference (if you're using a photograph) that's also greyscale.
The second image is a colour chart where I used a range of dark premixed tube colours then created my own chromatic black the the second column from the right. The decreasing values in the column are increasing additions of titanium white to the colour in box above it. The last column shows how I can increase the level of coolness or warmth by adding more of one of the primary colours used to create the base chromatic black.
Aside from learning to draw, the next best thing to know is colour theory. It will save you time, grief and money.