I've had a number of issues with the "Emphasis text" function to the point that it's unusable for me.
First, on Windows at least, clicking the dropdown and selecting a colour doesn't actually change the colour of the text. You have to select the colour then click the main button. That's very confusing (so much so, that I thought the whole feature was broken.)
Second, the biggest need I have for this feature is to de-emphasise the text. When adding point 2 to a slide, I'd like to fade out point 1. It would be nice to do this automatically, but failing that, creating two slides is OK. But because I don't have control over the colours, then the feature is unusable for me. I can select neither a light grey (if the rest of my slide was normal), nor the preferred colour of my text (if the rest of my slide was grey).
Third, when emphasising an entire line of a numbered/bulleted list, the numbers remain in the ordinary colour, which looks very odd.
An alternative fixing these problems (for me at least) would be to provide an additional feature whereby text from previous points would automatically fade when adding new points.