Until they get their eye in, a lot of art students find it hard to accurately portray tonal (value) differences in their art. In other words, the darks are not dark enough, the lights are not light enough or the mids are too light, too dark or not rich enough in colour.
Even experienced artists can lapse from time to time if they don't keep a discerning eye on the tones in their work. Some artists also find that it's harder to see the full range of tones as they get older - even more reason to keep tone in mind in the creation of any artwork.
Here are some quick pointers to not only correct the tone when needed but also teach (or remind) yourself how to see tonal changes:
1. Take a piece of white cardboard, draw a long rectangle and divide it into 10 equal sections. Paint the leftmost section white, and paint the rightmost section black. Mix a small amount of white and an equal amount of black together and paint the middle section this mid-grey. Then gradually paint in lighter greys as it goes towards white and darker greys as it goes towards black. You should end up with a scale, which progresses evenly from white to black.
This is firstly a good practise exercise to get familiar with relative differences. If you like to draw, it's a great one to do with pencils. But the real use comes in when you use it to measure the tone on your subject and compare it to the tone on your artwork. You may be surprised at the difference.
2. Make a black-and-white copy of your work (e.g. by photocopying the original, a scan or a photo) and really look at the tones. Are they really representative of your subject? Do they change evenly on round objects? Are your blacks and whites pure? Sometimes colour really distorts our perception of tone, so a black-and-white version can really give you a sense of what's really going on.
3. Learn to paint using the grisaille approach- a purely tonal underpainting in only one colour (hue). This will get the tones straight in your head before you add colour (or alternatively, leave it as a study to refer to as you paint the final work).
Tone is one of the basic building blocks in representative styles of art. No matter how beautiful the strokes or finish, without this solid foundation, an artwork won't come together. Poor tones can warp and image as you walk away from it, or mean that the artwork fails to capture the viewer's attention. Moreover, incorrect tones will greatly impact on believability, in many cases, even more than proportions. Concious measurement of tone and practise using the range of tonal values is key to successful works.