Choose custom colour for shape fill in Pages 5.2
First, be sure to change the fill type to Colour fill if the default template has Advanced Image Fill selected.

Then, as noted by @grgarside, select the colour wheel icon.  See the screenshot showing its location.  

From here you can select the sliders.  If you know the custom colour code, either HEX or RGB, then it can be entered into the color picker.  Otherwise, use the Eye dropper color picker to sample your color from another part of your screen.  For example, if you have your custom color in website, or other document, select it with the eye dropper and then it will paste into your object.