While in the Lab mode in Photoshop, it is a good time to create a base white or wet white. Select the Lightness channel, go to the top right corner of the Channel window, click the menu icon, and select Duplicate Channel. This will open a window where you can then name the channel, (white base or wet white), and check the ‘invert’ box.
At this point, a channel, which looks like a reversed film positive, is created. Double click on the channel and make the color white, select ‘Spot Color,’ and set the opacity to 80% for an underbase white or 25% for a wet white. For viewing purposes, create a solid 100% black channel to represent a black T-shirt. Place the channel under the new white channel you created. This will give the artist an idea of what the printed white plate will look like on a T-shirt.
Highlight the white channel and select “curves” or (command M). In the window, drag the left arrow to the right 8-10% to simulate a choke. For a wet white, drag the pointer to the right until the color is only in the areas of the image that are white and pastel colors. Make any adjustments, such as color balance, saturation, lightness, or filters now. Adjust the design with the curve to create nice transitions in the mid-range areas.