Hi Josep. I do a little photogrammetry too. You may have seen my tutorials on YouTube. I usually touch up the textures and model in 3DC so maybe I can help. Unfortunately for this 3DC uses vertex coloring so that means you have to have the vertex count very high in order to retain texture detail. To start I go into the Sculpt room and pick the Import tool from the left side. In the tool options I check the box for "import W/o Voxelization", otherwise you won't get the color. The pick Select mesh from that panel. and press the Enter key or Apply on that panel. If you switch to another tool you should see your model in full color.
if you switch to the paint room you can fully paint on it with layers. To export you will want to Retopo it, mark seams for the UV map, then Unwrap. Finally from the Bake menu choose "Bake with Normal Map". You can now hide the original model and in the Paint room you'll see the low res one you made. To export you can go to File > Export Objects & Textures.
As an alternate method you could choose to retopo / UV first, then bake and paint on the UV map instead of vertexes. I hope that was clear enough.