Right click slider and select Scale -> Style -> no markers
Right click Color ramp and select Scale -> Style -> all markers (top left pict).
Then you can edit color ramp scale range (top, bottom limits) and colors, including intermediate (right click on scale marker and select color).
Then hide ramp scale, show slider scale, match scales limits.
I would advise to play with color ramp separately first. It is in Classic -> Numeric.