The third colour workshop I taught was very much hands on. We talked about light, medium, dark, as well as bright and dull. Colour is the first thing people will notice about a quilt. But without good use of the fabrics' values, none will be able to determine the block design and all the hard work of matching points and seams will be lost in a blur of colours, much like a variegated afghan.
Value is so very subjective. It just depends on the fabric it is playing with.
A single fabric can be light in one block,
medium in a second block
and the dark in a third.
It is helpful to find the medium first. Determine the light and the dark and then fill in the gaps.
Everyone got a medium grey fabric to use as a reference point. Solids or near solids are much easier to place in a line when teaching the eye to discern subtleties.
The colour itself is a distraction so there are ways to eliminate colour. A red coloured piece of cellophane, plastic or glass can do this. Hold the cellophane to the eye and gaze at the fabrics from about 12 inches away. This can be very surprising sometimes.
Photocopying the fabrics on the black and white setting gives a permanent reference. A camera on the black and white setting also is beneficial.
A trip to a paint stores can provide the most useful tool. Find a paint card that has white to black. This is a professional gradient. It is quite helpful for getting a good range of lights, mediums and darks.
For me at least, value is challenging but the more I do the better I get at it.