You want to set up your page so you can start laying down color.

After you open your document, switch from the Layers window to the Channels window:

In this case, and in most cases, the linart was in Bitmap format. If that is the case, you need to switch to Grayscale before proceeding further. To do so, go to Image > Mode > Grayscale. Then, click Okay on the pop-up window:

The next step is to create a new channel to house your lineart:

You will be asked to name your channel. Type in Line-Art.
Make sure your Color is set to Black, and the opacity is at 100%:

Now you need to copy the contents from the Black channel to the Line-Art channel:

Now, click back on the Black Channel and fill the entire page with white:

You are most there. Now you need to switch to RGB mode for proper flatting:

Once in RGB, all you need to do is turn on the Line-Art preview and you are ready to start flatting.
To turn on the Preview, just click on the little Eye next to the channel. Don't click on the chanel itself, just the Eye: