There are a few ways to apply the LUT in the editor. Unfortunately there's no UI integration for this, so for previewing purposes, you need to use the Console.
Make sure colorgrading is active by default (it should be) with this CVar:r_ColorGrading 1
Load the new colorchart/LUT using this cvar: r_ColorGradingChartImage Textures/Colorcharts/colorgrade_cch.tif
Note that you need to give the full path with GameSDK as the root.
Open flowgraph, and add the node:Image:ColorGradient
For single player missions, you will also need to add a few nodes to set "default" colorgrading options. This is because the UI calls certain colorgrading LUTs when the player is hit, or when the player uses Focus.
The node is called: Image:ColorGradientDefaults
In the "TexturePath" input, add the full path of the LUT (just like in the Console example above).
Trigger it all on GameStart or something else.
The flowgraph should look something like this:
Now when you jump in game (or trigger it), the grade should be switched on.