I've been wresting with this question myself in redoing my group's world map. Outside of the Gaia shader, There doesn't appear to be a way to do this outside of creating a standard landmass, and then changing the coloration of the CC output, but I think I can live with having to do that.
I wrestled with trying to make ice caps in FT3. The solution I came up with (after getting ideas from Terraformer) is the Image Overlay feature of FT3.
Quick Tutorial on How To Make Ice Caps using Image Overlay in Fractal Terrains 3.
1. In FT3, Change Projection > Orthographic with Latitude 45 for a nice view from space of your work on the North Pole. (Later do this with Orthographic Latitude -45 when you work on the South Pole).
2. In your web browser, search Google Images for "ice". Choose any picture that is icy, blue and mostly white. Download the picture to your computer. OR in GIMP or Photoshop create a white JPG or PNG (or it likes BMP images) image size something like 5000 wide x 500 tall (a low wide picture will work, make it white with some light blue stripes or crosshatch if you like; it will probably be distorted to a circle, or stretched really far out, once applied as an overlay and viewed in a projection, so the ice cap will not look exactly like the "Ice" picture you choose in this step).
3. In FT3, click menu Image Ovelays > Show Overlay Window > ADD > Import Color Image >
Visible? Check Yes.
Always Show Overlay > optionally change it to "Show only if below water (Sea)". If you already have icy snowy ground in parts and you just want to add ice cap over the ocean parts.
Opacity slider > change opacity to about 85%.
Overlay Word Edges: LEAVE TOP 90, CHANGE BOTTOM TO 72, LEAVE LEFT -180 AND RIGHT 180 ! (For South Pole, use TOP -72, BOTTOM -90).
CLICK IMPORT COLOR IMAGE > Browse to your "Ice" picture.
USE TOP LEFT PIXEL AS TRANSPARENT? SAY NO.
The "Size (samples)" settings can be anything, the program may set it automatically, try Width 5000, Height 1000.
Now you should have a smooth round North Pole Ice Cap.
If you want to change the Transparency (Opacity), or change the Bottom Latitude of how far the ice cap goes down the globe, then go back to Image Overlays Window, and click Edit.
Now you want the ice cap to have non-smooth edges? I find a way to make jagged edges by hand. I did not find a way to make Fractal edges yet.
To make Jagged edges now by hand,
4. In FT3, Change Map Projection to Miller Cylendrical.
5. Image Overlays > Overlay Erase Tool.
View > Toolbars > Selection Options > change Eraser Brush size to Width 111, Height 55.
Now start erasing the straight bottom edge line of the ice cap! Erase up-and-down making some jagged lines.
6. In FT3, Change Map Projection back to the projection that you like. The jagged lines will look quite different in other projections. The jagged lines should extend all the way around the ice cap, in projections that produce circular views of the North and South.
EASIER THAN IT LOOKS WRITTEN OUT -- if you're like me, and you heard "Image Overlays are complicated", don't worry, it's not complicated to do this. It is easy to turn off the overlay if you messed up. No harm to your world. No slow delicate painting of climate and temperatures affecting the fractal modeling and taking time to process.
This could be a way to learn how to do an Image Overlay in FT3 -- not saying I'm right, I'm asking for tips, because this is absolutely my 1st world in FT3, my 1st image overlay, and the 1st time I've presumed to post a tutorial here on the helpful Cartographer's Guild. I don't know if Ice Caps was an intended use for Image Overlay, but it seems to be a workable alternative method, independent of the Temperature model, Altitude Color, and Image Climate selections. I noticed other people had asked for a way to achieve something like this in the past.
EXAMPLE IMAGES (my first fractal terrain world & my first novice "map" posting on Cartographer's Guild -- still keeping most of my world hidden for now, it is a WIP Work In Progress)
IMAGE A - my Ice Cap made in FT3 with Image Overlay and Overlay Eraser. Note my ice cap is roughly circular in extent but jagged edges, over the seas; much of the white at lower latitude is from cold high mountains in northern climates.
IMAGE B - Compare with the cool looking icecaps in FT's Gaia Shader, that everybody wants when using Image Climate shader, Altitude, Climate, and Terraformer shaders.
IMAGE C - Fractal Terrains 3 Image Overlay settings for Import Color Image (import your white icy pic at the arctic polar region)
What a great tutorial! Thanks for posting it. I can't seem to get the erase brush to work properly...don't know why but it doesn't want to erase the overlay at all. It worked for a very short time and then stopped!
Thank you. Good luck with the eraser. I'm not sure what the problem might be. I would Save your map, restart the program, and see if it works again. Also make sure your Tool properties view is open and adjust the eraser size. I have had trouble like you described with other painting tools in FT3 (raise, lower). The tip I've heard is to do short strokes. However sometimes it seems more effective if you hold the mouse and linger in a small area. However for me the erasing worked smoothly enough with tool Width 111, height 88, or so -- a big brush, and the Miller projection, I was able to erase broad swaths fairly quickly. Another idea for you that might or might not help, you could make your viewing window smaller. I think it may process the image faster when the display of the world is smaller on your screen. The main tip is restart the program. I've only done the erasing technique once so far myself. Still working on my world. That would be cool if you are able to produce something from my tutorial tho.