My drawing is essentially rectangles inside of a larger rectangle at this point (railroad passenger car). I was working on a layer to extrude when all of the previous work went red borders.
Hi TwinStar and welcome to the forum.
Are you clear with the workflow? (I don't want to be insulting, but it happens often for people coming from 2D packages, I don't know anything about Sketchup, that there are problems getting used to the 3D workflow. But once you have caught on you will not look back. I say that from my own experience.)
For example 1) make a
sketch to define your 3D objects outlines -> extrude that so you have a 3D object -> project a
drawing from your 3D object.
Note that red in a sketch means that the lines are not completely constrained, red in a drawing means that the corresponding features of the drawing no longer exist on the 3D part. That can happen when your 3D modeling changes the numbering for the edges, for example.
If you already have a drawing finished of a 3D part you want to modify, be sure that for existing features that you modify the existing, but don't delete and remake them. In the first case the feature numbering remains, in the second the features will have new numbers and will not be found by the drawing, causing them to go red in the drawing, even those things that you have remade exactly as they were before.