Bokeh color mapping behaves as follows :

```
x < 0 : 'red' # values < low are clamped
0 >= x < 33 : 'red'
33 >= x < 66 : 'green'
66 >= x < 99 : 'blue'
99 >= x : 'blue' # values > high are clamped
```

This is easy to implement and works as expected. However, is there a way to explicitly define a color for the clamped / extreme values which will be off the color mapping given? e.g:

x<0 : ‘black’

99>x: ‘white’

that is whilst keeping the cmapper for the middle range.

thanks