Creating white corridors on a blue background.

If I wanted to create some dungeon maps like the old D&D modules with a blue background and white corridors how would I create the corridors once the entire map is made blue? I am fairly new to roll20 and love the gaming site.

Here is some things I found when I googled Old School D&D map: Campaign Cartographer has recreated the style, but its hard to use and expensive to buy. :( <a href="http://www.profantasy.com/annual/2007/december07.h" rel="nofollow">http://www.profantasy.com/annual/2007/december07.h</a>... There is a youtube series of videos on how to use these with CC. <a href="https://www.youtube.com/watch?v=qxmIeNvWSxg" rel="nofollow">https://www.youtube.com/watch?v=qxmIeNvWSxg</a> Looks like Dungeographer does them also. Its cheaper and probably easier to use. I've never used it. There is a free version you can try. <a href="http://www.dungeonographer.com/wp-content/uploads/" rel="nofollow">http://www.dungeonographer.com/wp-content/uploads/</a>... This one has potential <a href="http://davesmapper.com/" rel="nofollow">http://davesmapper.com/</a> Here is a detailed tutorial on how to create them in GIMP which is a free drawing program <a href="http://www.cartographersguild.com/tutorials-how/24" rel="nofollow">http://www.cartographersguild.com/tutorials-how/24</a>... Symbols <a href="http://www.flickr.com/photos/drzeuss73/5921780958/" rel="nofollow">http://www.flickr.com/photos/drzeuss73/5921780958/</a>

And there is this in the marketplace, not exactly the same, but good. <a href="https://marketplace.roll20.net/browse/set/252/old-" rel="nofollow">https://marketplace.roll20.net/browse/set/252/old-</a>...

Konrad, thank you very much for the detailed response and the time you put into the information.