Here are some guidelines for laying out a high-visibility color scheme for an RC airplane or glider.  These guidelines were formulated in a combined slope-soaring/ thermal-soaring environment, where the aircraft is often below the pilot's distant horizon, or below the ridgelines of nearby hills, yet at other times is engaged in prolonged circling maneuvers at high altitudes and at very great distances.  They'll be useful for increasing the visibility of RC airplanes or gliders in many other situations as well.  The goals of these guidelines are as follows:

 

1) To prevent the pilot from completely losing sight of the aircraft when flown at extreme distances against a wide variety of backdrops including terrain, bright clouds, dark clouds, and blue sky, even in very low light conditions such as twilight

 

2) To allow the pilot to quickly judge whether he is seeing the top surface or the bottom surface of the wing, so he will not lose his sense of the aircraft's orientation during circling flight at extreme distances, and also to facilitate a fast assessment of the aircraft's orientation during aerobatics. (This is especially important in the case of a flying-wing aircraft like a Zagi, where the aircraft's silhouette offers few distinctions between a top view and a bottom view!)

 

3) To prevent the pilot from losing sight of the aircraft when it is flying directly toward the pilot during the approach to landing, when it presents a very small frontal area.  This situation typically presents the greatest challenge in cases where the landing approach path passes below the pilot's skyline--e.g. below the ridgeline of a nearby hill, or below the top of a treeline.  The challenge of keeping the aircraft in view under these conditions--as well as the urgency of the task--is dramatically increased if the pilot has flown deep into the twilight, and needs to land the aircraft quickly before all light is lost.

 

4) To help the pilot find the aircraft when it accidentally lands in deep brush, tall grass, etc..

 

These guidelines were not aimed at meeting some other, perhaps equally legitimate goals, such as: maximizing the model's aesthetics, or providing strong visual reference lines to enhance the appearance of aerobatic maneuvers, or providing a scale-like color scheme.

 

Here are some guidelines to meet the above goals:

 

1) A simple 2-color scheme is better than a flashy "rainbow" design with many different colors.  One of the two colors should be quite light, and the other color should be quite dark.  The best choice for the "light" color for overall visibility--including visibility against grey clouds or dark terrain in very deep twilight--is either white, or a pale day-glow fluorescent yellow-green.  The best choice for the "dark" color--to allow the aircraft to create a strong, dark silhouette against a bright blue sky, or against bright white clouds--is black.  For aesthetic purposes, many pilots may choose to use other pairs of light and dark colors, but for simplicity here, we'll just refer to the two colors as "white" and "black".   If you've ever watched a bald eagle or osprey soar in the company of another, drabber, bird such as a red-tailed hawk, you'll appreciate the striking way that blacks and whites stand out against a variety of backgrounds, even at very great distances.

 

2) Most of the bottom surface of the aircraft should be black.  Most of the top surface of the aircraft should be white.  This will help the pilot to quickly judge whether he is seeing the top surface or the bottom surface of the wing, even at great distances, or during fast aerobatic maneuvers.

 

3) 20% to 33% (1/5 to 1/3) of the bottom surface of the aircraft should be white.  20% to 33% of the top surface of the aircraft should be black.  This is so that the aircraft never presents an all-white or all-black surface to the pilot, which could cause the pilot to lose sight of the aircraft against some backdrops. 

 

4) It is very important that the contrasting color (as described in guideline #3) be applied in one or two large, solid, concentrated patches, rather than dispersed into a collection of trim markings, stripes, etc.  For example, the contrasting color could be applied to the outboard parts of the wings, or alternatively, to the center section of the wing.  The contrasting color is much less effective if it is dispersed into multiple small patches, or long, narrow, rectangular shapes.  (Examples: here's a photo of a model with an upper surface that is primarily white, with large black areas at the wingtips. Note that if the black areas had been applied at mid-span rather than at the tips, the black portions would have still presented two strong, concentrated, blocks of color, but the white area would have been broken up into three different blocks rather than presenting one, concentrated, highly-visible block. Here is a photo that shows the undersurface of the same model. Although the light is poor, the viewer can see that the fuselage and the center section of the wing form one solid block of white, while the rest of the undersurface of the wing is black. Note the great difference between the appearance of the model as seen from above and as seen from below. Note also how the black silhouette stands out against the grey sky. Here is a photo of a model that is aesthetically quite pleasing--the color scheme emphasizes the graceful, long, span of the wing--but the black areas comprise such long, narrow, rectangular shapes that at great distances they will tend to be lost to the eye and will not do very much to increase the model's visibility.) 

 

5) The model's color scheme as seen in a side view is less critical, but a light color will be helpful during aerobatic maneuvers below the pilot's horizon, or below the ridgeline of a hill, or below the top of a treeline.

 

6) At least 50% of the wingspan should have a white leading edge.  This coloration should wrap all the way around the leading edge, all the way down to the flat part of the undersurface of the wing, so that the white area is visible to the pilot even when the aircraft is flying directly toward the pilot in a nose-high pitch attitude.  The idea here is to ensure that the aircraft is not lost from view when it is flying directly toward the pilot under the most challenging set of conditions--during the approach to landing, against dark terrain or trees, in low light levels. An aircraft with dark leading edges--or with a dark wing undersurface that extends all the way forward to the extreme "nose" of the airfoil--can be extremely challenging to keep in view during a landing approach against a dark backdrop.

 

Having said all this, I'll also add that I've had very good results with a simple, translucent orange, single-color scheme on my Gentle Lady RC sailplane.  The color tone is apparently dark enough that the model isn't easily lost from sight against bright sky or bright clouds, even with the light passing through the translucent wing panels.  The orange also stands out well against dark clouds or terrain.  If a bright blue sky is darkened with brown, polarized sunglasses as described elsewhere on this website, then the translucent wing orange surfaces--backlit from above--become strikingly luminous against the dark background of the sky.  (Brown glasses preferentially transmit orange and red tones).  The Gentle Lady has such a distinctive polyhedral layout that maintaining orientation is seldom a problem, even at great distances.  Also, the model flies slowly enough, and is stable enough, that temporarily losing one's orientation does not create as urgent a problem as it would with many other aircraft.