Well Soundman, welcome to modding!!!
Now that you have gotten your feet wet, there are a couple of programs you should have. I'm not sure what PhotoShop version you have (and I'm no expert either) but the plug-in that works for me came from this
web site. Try to match up the nVidia plug-in that best suits your PhotoShop version, download and install.
While your there also download the WTV image viewer and put the .exe shortcut into your "Send To" folder. Then when you are interested in looking for that specific image, just right mouse click the file, open the "Send To" menu and click on WTV. The particular image, and all other .dds images held within the folder can be seen.
You have the DXTBmp converter which I also have in the "Send To" folder. I have used this tool for doing simple paint and draw changes to .dds files. Either with the main image or the Alpha black and white image which is an important aspect of the .dds image. The alpha channel image is of course a mirror image of the main image. The black part of the alpha blocks whatever is visible on the main image. The white part is translucent, it allows the image to come through. The whiter the alpha the darker the true image will be when it is processed.
Really the same is true of the black parts, the lighter the black (like shades of grey) the more of the concealed image will show through. So if you put something onto the main image, like a flag or text, the addition on the alpha channel must be in the exact same position. AND, depending on how bright or dark you want the new addition to be depends on how white or dark the alpha channel specific position is.
That being said, what I've usually done is export a copy of the alpha from the image file (both .dds and .tga files have alpha channels) and work with them in PhotoShop. I've adjusted contrast to get the right translucent or blocking color that I want, then import it back to the main image using DXTBmp.
Again, I like using PhotoShop for most detail work with either the main image or the Alpha channel. There is a lot more you can do with an image using Photoshop over DXTBmp, but the export and import feature for the alpha channel is the easiest using DXTBmp.
So, to lighten the periscope lines you
could use the DXTBmp to work with the alpha channel image. Send the "Alpha to the Editor" and try to lighten the lines you want with the whitest color, and as small a line as possible. For a much better job export the alpha to PhotoShop and redraw the lines with a brighter white color, then import the .bmp alpha back into the periscope_mask...... file with DXTBmp. OR if you want a slight green color, use PhotoShop to color the "main" image lines and save the .dds image using the nVidia plug-in which should automatically open up when you ask to save the image as a .dds. The nVidia DDS Format Screen will appear, choose "DXT5 ARGB 8bpp/interpolated alpha" and "save". Put the image file back where you got it and run the game to see what it looks like.
OH, by the way, before you mess around with your files,
ALWAYS use "copies" of the original so you can go back to them if your work doesn't come out right.