I have heard modelers grumble about the results, when taking model photos with plain backdrops, such as light blue sheet of cardboard. It doesn’t really look right, they say.
The problem is really easy to fix, if you have, or have access to, Adobe Photoshop. The answer is to correct the sky blue to a color you like, then make it more realistic with a gradient. I’ll illustrate. Let’s say you have a “photo bench” shot like this, and the background either was white, or got washed out. It looks a little odd. (Likely you recognize the center car here as a cast-metal Ulrich gondola.)
Photoshop has a tool called the “magic wand,” which can select all areas of the same color. You can choose contiguous-areas only (often the right choice for sky), or for non-contiguous, that is, all areas throughout the image which are that color. You can also set the “tolerance,” meaning the range of color the wand can select. In the photo above, I just chose the white area at top.
Once you choose the sky area, select a blue that appeals to you, for example in the Photoshop “Color Picker,” and make it the “foreground color,” as Photoshop calls it. Then fill the selected area with that color. It’s better but still doesn’t look natural, as you can see here.
I haven’t refined this image by blue-converting areas under running boards, etc., as I just want to show the main approach.
What this sky needs is some fading toward the horizon, as natural skies almost always do. Again, there is an easy way to fix this in Photoshop. First, select a lighter blue as the “background color,” which will be the second fill color. Now select the gradient tool, and simply use the mouse to draw a vertical line in the sky area. This makes a continuous gradient between your foreground and background colors. You can vary where the line starts and stops, so the gradient is in the area you want. Experiment to see how it works. Here is one result.
This is already a much more natural look, especially compared to the almost-white sky I started with.
The technique isn’t limited to a posterboard kind of sky. Any sort of photo can have its sky area improved with the same tools. Here is an old shot of mine with a commercial backdrop, but with the sky washed out. The SP gondola is a kitbashed Athearn model.
Again, I filled the selected sky area with blue, then used the gradient tool, only within the sky area, to get some fading. Here I only faded just above the hills, because the true line of the horizon is much lower.
This photo, from an old slide, is kind of washed out and doesn’t have great color to start with, but notice how much better it looks with the sky modification.
This technique is quick and easy, and if you don’t have enough familiarity with Photoshop to know what tools I’m talking about, most any aftermarket book on the application will show you how to do this and many other adjustments.