1st, if you're going to change them in the Layer Manager, you don't have to do them one-at-a-time, just select all the layers and click on any one of the color buttons.
2nd ... DON'T DO THAT
LT has Viewport overrides and Layer States (at least I think LT does) My recommendation is to make a full size viewport, and work in that, you won't even be able to tell you're not on the Model tab unless you look at the Layout tab. One of the big advantages of working in viewports is the ability to display layer colors. linetypes, lineweights etc any way you choose on a per-viewport basis without altering the model. If you need any additonal help creating a viewport and figuring out how to grey all the layers except your 3 working layers, just ask.