Color Scheme: trim near-mean values in a color scheme

From John Griffiths:

There is an option to 'trim marginal values' for linear gradient colour schemes in the 3d brain activity visualizer. What would be more useful would be a 'trim near-zero' values option, so that values some configurable distance to zero are not shown with colours.

The reason why I think this would be useful is that often these plots are swamped with colours; it would be handy to be able to filter out the lower amplitude stuff so that the 'big activations' are emphasized more clearly.

After discussing with Marmaduke and Tim (9 Nov during their visit to Cluj), this need was emphasized.
It could prove useful for the visualizations that Francesca needs for the mouse experiments.
The need is more exactly to trim some values inside the interval (around the mean value) - with an extra double slider - cutting the inside of the 2 slider positions.