I've been playing with PC train simulation software for years, and I get a lot of enjoyment out of it. I've also been interested in railroad signals for years. I've learned quite a lot about them, and I'm still learning more all the time. Naturally, I was excited to see MSTS's signals in action when I first got the game years ago. As a result, the first thing I noticed in the game was the choice of signal lamp colors.
For "as real as it gets" it wasn't very real!

It has now been fifteen years since the game came out, and the un-realistic signal colors still plauge the game's content today!
Is this still relavent? Yes, a game that old is still used. Especially when its content has a formidable replacement host.

Even payware routes are not immune to this problem. Many game users might not notice the in-game signal colors. But if you have spent any time staring at real signals (as most railfans do), then you might have noticed what I noticed.

I can demonstrate what I'm talking about in the most drastic way imaginable:

On the left you see the original photograph. On the right, the MSTS default!

The signal lamp shown is a LED signal lamp, so its appearance is both very bright and very uniform in hue. We still must keep in mind that the color mixture will be perceived differently by the eye than by the camera and your computer monitor.
But, it is still obvious how much blue the real signal lamp has in it! The green color used by default for MSTS signals has none.

This problem can take as little as 5 seconds to fix!
Everyone can do it! In a very short time you can even have all your routes corrected. When you notice it on a route, here's how to correct it:

If you wish to spend more time on it, you can adjust the other signal lamp colors as well.
If you choose to make more than the simple edit outlined above, you should consider backing-up the sigcfg.dat file prior to editing it.

A route's sigcfg.dat file specifies signal lamp colors in the LightsTab group, with each color in a LightsTabEntry entry. Each entry assigns four color values to an alias (its name, like "Green Light") that is referred to further in the file. The four color values are specified as integers from 0 to 255. In order, each color value represents:

alpha  red  green  blue

This is the same as the 32-bit color specification common on computers today which many users will already be famliar with. Previously, I stated here that you'll want to use a fully-opaque value of 255 for the alpha component. I've since gotten creative, and have experimented with this value. I now prefer semi-transperent values for alpha. I've also darkened the colors in general. I believe this makes for a more-realistic appearance. Note the values in the table below.

Here are the NEW color values I prefer to use, depending on the type of signal:

Use the Colorlight values for all types of signals except those otherwise specified above.
Note that, for signals I wish to give an older and dimmer appearance, I may scale down all four values even further.
I do not recommend an alpha value below 128, however.
Original PRR Position-Light signals should use the Fog Yellow color.

Note that Open Rails renders signal lights differently than MSTS, so they will always look different amongst the two platforms.