Dark values carry a sombre feel while light colours give a feeling of peacefulness. If you want to create a focal point, surround a light area with a dark one, it will create a frame that draws the eye like a magnet. Used in conjunction with texture, colour and line, interest can be heightened and intriguing images created. In his picture Temple Gardens, Paul Klee uses value to draw your eye around the painting.
A grey scale image makes this clear, as your eye dances around following the black shapes or the light ones.
In quilting, then, the key is finding enough pieces of fabric in your chosen colour to create the values you "need".
Edinburg Sunset, challenged my stash, and as I have 5 large boxes of fabric and odd pieces tucked here and there in drawers and on shelves because the boxes are too full, I just couldn't explain the need for more fat quarters when all that was needed was a tiny square or two.
Fortunately I have a wonderful tin of Inktense pencils and a selection of textile paints.
The pencils were used to darken the fabric for the windows. To use them just colour, then brush with a barely wet paint brush and iron once dry to set the pigment (protect your work surface with a piece of paper or plastic sheet). Then the fabric can be bonded using any bonding agent of your choice. The textile paint was used to create the orange red sky fabric, foreground and windows on the dark house. A good range in value can be achieved by dilution of the paint with water (again protect your work surface), or more than one coat. Lightly sprinkling with salt while it's wet will create texture, if it is left to dry without moving. Again heat set the colour with an iron set for the fabric, in both cases I protect my iron with baking parchment. As both paint and pencils blend and mix with water, you only need a few colours to create a wide range. I would recommend this as an alternative to buying more fabric if small pieces are your passion and you enjoy a little colouring.