No question, first asked, is ever stupid. We all gotta start somewhere.
As @paperdigits remarked here some time ago, a color look up table (CLUT) or just “look up table” (LUT) embodies specific directions to transform an image’s color. The acronyms have their origins in an arrangement of colors as a table that can span three spatial dimensions: a cube. Characteristically, the additive primaries, red, green and blue, are inducted into the rôle of axes to a color space. The introduction of the idea that color is a “space” permits the expression of “color movement” in terms of shifts in red, green or blue directions. In the usual course of events, some agent examines a pixel in a color image; looks up that color in the CLUT and obtains instructions to move that color some distance elsewhere. Such movement is commonly small, but it need not be. CLUTS can rearrage image colors subtly or as a whack to the head. You may see the term “LUT3D” bandied about. The reference has little to do with 3D modeling software like Blender; its just a reference to this cubic arrangement of color information.
Since 2019, or so, Darktable has possessed a module, lut3d; users may load CLUTS from a variety of file formats (coming from any number of sources) and apply the specific color transformation directions embodied in the CLUT. G’MIC-QT plug in users may similarly apply CLUTS to their images through such filters as Apply External CLUT (get the CLUT from a file), Apply From CLUT Set (Load a CLUT set and use one of its CLUTS, referenced by an index number) Color Preset (Choose a CLUT “color preset” from any number of pre-installed packs) or Simulate Film (Choose a CLUT specifically designed to shift colors toward those of a Storied Photographic Film from Olden Tymes — yes, great grand child: at one time, photographers loaded photographic film into cameras! As a young photographer, I took some of my best pictures on the slide…).
Noted in passing, above, G’MIC-QT filters Customize CLUT and CLUT from After - Before Layers permit users to create CLUTS, should all of the 800 or so available CLUTS in the G’MIC-QT plug-in fail to float the boat. The estimable @paperdigits, again, delved into Customize CLUT in a way that I shan’t disturb. For purposes of this thread, however, CLUT from After - Before Layers offers itself as a solution to the problem of devising a “mood CLUT” — one designed to shift an image’s colors to emulate the morning or evening time ( The Blue Hours ) or a season (cold, grey Winter; bright, chromatically alive Summer!).
The game is to harness whatever color adjustment tools that a platform offers to nudge, drag or trash an image into a new color locale, whilst saving the unaltered “base image” in another, lower, layer (say “B” for “Before”). Locate this unaltered base (“Before”) image in the lower layer. Locate the thoroughly color-trashed image in the top (say “A” for “After”) layer and then invoke the CLUT from After - Before Layers, making adjustments something like those illustrated below, saving the same to wherever you might be inclined store CLUTS (here: Output Folder happens to be “shm”).
The key takeaway is that all the multitude of color adjustments made to arrive at the image in the After Layer become embodied in the CLUT. You may apply all those adjustments again, in one fell swoop, to another copy of the Before image — or to any image. So, if by trial-and-error, color adjustment after color adjustment, you’ve arrived at the perfect (for you) “Morning Mood,” generate the corresponding CLUT and save it out. Pro tip: after saving the CLUT, reverse the images in the layer stack and generate a second CLUT from the reversed pair. That becomes an “inverse CLUT” which works in the opposite direction, color space wise, to the first CLUT — an “Un-Morning Mood” CLUT that (potentially) could make rather too morningish images less so.
This should be enough to start having fun. There be tutorials that I need to be writing…