A grayscale image is converted to RGB and coloured with blue positive,
red negative, and white in between.
| Bimage* p | image. |
| float red_min | beginning of red gradient (most negative). |
| float white_min | end of red gradient (fade into white). |
| float white_max | beginning of blue gradient (start with white). |
| float blue_max | end of blue gradient (most positive) |