Provide a custom color scheme following one of the two options below:
Option 1: custom gradient, one line for each value-color pair in the format "value color" (the other values will have an interpolated color) e.g.:

2.345 #00ff22
4.556 #aa3344
5.667 #fafafa

Option 2: value classes, one line for each class in the format "minvalue maxvalue color", e.g.:

1.0 2.0 #00ffaa
2.0 3.0 #fd9000
3.0 3.4 #aaaff0