1.) I started with this photograph of a lighthouse that I came across somewhere, and decided to add a little bit of non-realistic color to it.

2.) Opened in the GIMP.

3.) Since I'll be adding color the just the sky, it has to go behind the foreground (the lighthouse and the grass) and on top of the original sky. Therefore, the first thing to do is to use the select by color tool to select all you can of the blue sky. I found that a threshold of 20.0 was easiest to use in this case, but find what's best for you. The first screenshot is after clicking somewhere in the top left, the second is after three more clicks (don't forget to use the shift key to add to the selection).

4.) Notice that a few of the thin brown stalks of grass are selected in the previous step. I decreased the color threshold to 3.0 to deselect some of the bigger ones, still using the same tool, and the ctrl key to remove from the selection. Remember that in GIMP, selecting is an action, which means that if you mess up you can just undo using ctrl+z.
Note: at any time, use right click > Select > None to deselect everything.

5.) For me, part of the sky got deselected with the stalks, and notice also that a tiny bit of my lighthouse did get selected. If this happens, just switch to the select hand-drawn regions tool and select (shift) the sky and deselect (ctrl) the front of the lighthouse. Don't worry about being perfect.

6.) Wait a minute. What we were trying to do was separate the sky and foreground so we could get something between them. Whatever goes between, the foreground has to be on top of it, not the sky. So right click on the image, Select > Invert to select everything but the sky. The reason we started with selecting the sky was because it has a smaller variety of colors, and is therefore easier to select using the select by color tool. Ctrl+c to copy all of that, create a new transparent layer (call it something creative like 'foreground', and ctrl+v to paste. Then anchor the pasted layer down by right-clicking on the 'Floating Selection' in the layers box and selecting 'Anchor Layer'.

7.) Create a new layer for the colors, and put it between the background and foreground. Click on it in the Layers box to make sure it is selected for the next step.

8.) Render plasma as shown in the first screenshot. Pick something you think looks good, although I tried to use not too much blue and not too much neon-ish green or yellow, because those might look weird against the sky later.

9.) The plasma comes out very un-smooth, so the first thing to do would be to blur it a little. A Gaussian blur with a radius of 40 pixels makes things a lot better.

10.) I decided to use the random colors just in an oval around the top of the lighthouse, as opposed to on the entire sky. Use the elliptical selection tool to select an ellipse where you want most of the color to be.

11.) Quickmask the selection, and apply another Gaussian blur to it (this will blur the selection) with a big enough radius to have color end up even on the left side of the image. I used 250 pixels. Switch back to a selection. Nothing really visible will have happened because the border of the selection is still pretty much the same, but now the pixels each have a "percent selection" of sorts, where for example one pixel could be twice as "selected" as another.

12.) Now the oval is selected in a nice and faded way. What we want to do is fade out away from the center of the oval, or in other words get rid of the color outside the oval and leave what's inside. Invert the selection like you did with the sky, and right click > Edit > Clear.

13.) You see that sharp white cloud just left of the lighthouse? That's left over from the sky, in the foreground layer. You may not have to do this, but to get rid of it, hide all layers but the foreground and use the select by color tool (I used a threshold of 10) and Edit > Clear. Show all the layers again.

14.) Some of the colors, especially the yellow, are a bit extreme at this point. I reduced the opacity of the colors layer to 85% to let some of the original blue sky through.

15.) I wanted to add some streaks of white radiating from the top of the lighthouse. These would go behind the lighthouse, and in front of the sky colors, so create a new transparent layer and put it there.

16.) They way I chose to do this was by "smearing" blobs of white outward from the lighthouse. Go ahead and zoom in to the top of the lighthouse, and use the ellipse selection tool, the shift key, and prossibly lots of ctrl-z to select some circles, in a circle. Look at the second screenshot.

17.) Use the fill tool to fill the circles with white, then deselect them.

18.) Put your mouse in the middle of the ring of circles, and read off the pixel coordinates from the bottom left corner of the window. Make sure the layer to hold the streaks (currently with the circles in it) is still selected, then apply a 'zoom' motion blur centered at those coordinates.

19.) Looks all right so far. I brought mine down to around 80% opacity, so they weren't too glaringly white. Personal preference.

20.) An 0.5-pixel Gaussian blur on the foreground got rid of some noise and hard edges. Not much else in this case.

21.) Create a new transparent layer for the border and put it all the way on top. We'll do the border similar to how we did the faded sky colors, using a blurred quickmask.

22.) Select the whole image, then shrink the selection by a few pixels to leave room for the fade.

23.) Switch to a quickmask and Gaussian blur it.

24.) Switch to selection, invert it to select where you want the border to be (so far, the image part was selected and not tbe border), and fill it with white.

25.) The original again, the final product, an image of the layers in the final, and my final result as a GIMP XCF file.

26.) Another image that I made the same way, with different values. Looks slightly better in my opinion.