Color Preview & Picker
If you are in any file, of any type, that contains something that looks like an RGB hex value, rgb(100, 100, 100), hsla(0,100%,50%,0.2) etc., just hover over the code and a popup will appear that shows the color preview.
This works for gradients as well.
If you move your mouse into the color preview area and then click, a color picker dialog will appear that allows you to choose a new color. You can drag things around as well as edit the values directly.