you can make it any color you want, just make sure it's the same color for all pixels that have to be transparent (best use the pain bucket to fill these) and that this color is not used for any terrain.
I recommend using the image-viewing program "IrfanView", it is for free, very easy to handle and very flexible concerning the span of formats (it can open images, videos and adio files).
just open the prepared image in it (all transparent pixels colored in the same color, best use microsoft paint or gimp for it) and then go to File -> Save as -> then enter the name and select .png as the format and make sure to check the little box at the top right that says "Save transparent color" or something similar. after that click "Save" and a small window will pop up where you will be asked what colour shall be transparent. click on taht color and the image will be saved automatically with this color set as transparent. you can open it again in irfanview and you will see that the transparent pixels now look black, this is because black is the default background color of irfanview itself so it really shows you that these pixels are transparent.
looks nice, better than the house and the floating islands
but I think the left and right borders of the map look too much like they are cut off, you should make it a little more round and "repair" that tree
Edit: maybe you should also put a .png version of your maps into the download-files. I think this would be better to have a quick overview over the map because one would not have to start CC but could have a look at the map in any image-viewer-program. furthermore .png images can also used as maps in CC if you set a specific color as transparent in your image-editing-program when saving as .png.