(Advanced labeling) setting intervals in symbology changes the color scheme

[StephenP]

I'm editing the symbology of a raster file and I've noticed that when I use the Advanced Labelling function to add another interval (from 1 to 2) it changes the colour scheme to a much darker palette. This only seems to happen when setting it to 2 intervals. See screenshots below, this is before and after I hit apply. Any idea why this is happening and how I might fix it? This happens with every colour scheme I choose.