Now, obviously these results look pretty wonky, but they're a good start. I like how the light yellow that will be my range of hills/mountains has a sort of curve to it now, and the reshaped lape in the top left corner is good too. Once you get something you're mostly happy with, it's time to rinse and repeat - run crystallize again, only this time lower the settings by at least 25% - I'll start with a setting of about 30 and see what I get. Again, bump the settings up and down just a single notch at a time - I found that 29, 30, 31 and 32 all gave me drastically different results, but ultimately 30 is the setting I kept.
Run crystallize again. See if it does anything you like while keeping the setting at 30; if not, drop it again to about 20 and see what you get. Each time you lower the settings, your edges are distorted with a greater level of detail, and more of the original shape left undisturbed. As you work your way down, you'll gradually get some plausible looking coastlines, boundaries, possible river tracks, and so forth.
Starting with the picture above, I ran Crystallize again using settings of 23, 15, 15 again, 9, 6, 4 with a reseed, 7 (I was curious), and 3. The minimum settins is 2, but I didn't feel a need to go farther so I stopped.
To be honest, I usually only run Crystallize half as many times as this, but I was having fun. As long as you eventually get to a setting around 4 to 8 as your final stop, you'll be fine.
Step 4 - Fill in the gaps
So, the white gaps in the original grid scribble have also gotten "crystallized" and are now some intriguing shapes in between our selected color patches. If you look closely, you'll also see a few places where the crystals overlap, giving you semi-transparent or color-blended zones. If you've gotten the Adjustments > Transparency plugin, now is the time to use it, increasing the setting to "fully opaque". Since we use the flood fill at a low tolerance, those translucent spots can be a pain.
Next we're going to use our color picker, possibly our magic wand, and our flood fill to take care of recoloring the gaps.
As with the rest of this tut, the theme of the day is Play Around. Should I bridge those two dark green zones in the lower left with more of the same, or extend my mountain terrain between them, or spread the lighter green of the top left down into those spaces? The program has already given me some new "in-between" colors to play with thanks to the blending of multiple crystals; maybe I'll want to use those to fill convert empty spaces into "in-between" zones.
So. If you don't want to create another color from scratch, use your color picker to select something, set your magic wand tolerance no higher than 33% and use it to outline the area you want to fill. You actually only need magic wand if you want to fill in more than one area at a time; press and hold the Shift key while you select your zones. If you find that the magic wand is selecting stuff you didn't want it to, just drop the tolerance and select the area again.
Assuming you used magic wand, once you've selected as much of the zone as you want, use flood fill. If you want that color in more than one place, set the flood fill tolerance to 0 so you don't cover anything by mistake, and go to it.
Okay, as far as this technique is concerned, you're done. You've got nice wiggly borders and boundaries, areas of color blend that overlap the main zones, and hopefully a map that is throwing suggestions at you left and right. For myself, I can already get a good idea of where my rivers are going to go based just on this.
An option, if you want to continue, is to open a new layer, and render some clouds on it. Set the layer's blend mode to overlay and you'll get a whole new range of texture - although admittedly, if you're going for a political map, the clouds likely defeat your purpose. If you do use them, they can suggest new elevations and possible paths for hills, rivers, roads, and different water depths. If you do that, though, I'd suggest only using one color for your water when you're doing the scribble step - otherwise it just looks weird once you try to use the clouds over the water.
Considering we started with a sloppily-colored grid...
...this doesn't look too shabby, now does it?