There is no single answer. First are you talking visually or electronically? Then it will depend on what you have covering your eyes and the type of canopy on your craft. And then there is the type, design and colors of the airplanes. The height above the surface will have an impact on the air density. The type of terrain the target plane has as a background will have an impact. And the contents of the sky with regard to density, smog, fog, smoke etc. and the type density and color of the clouds will also have an impact. Well I could go on but visual air to air, combat, is pretty much a thing of the past. The AWACS and more modern air control plans today have electronic awareness of the combat area and control the planes from miles away and the fighters are cleared to fire from miles away.
Or is your question about the WWII bombers and fighters? WWI combat pilots were for the most gentleman, but that is another story.
By the way I am retired USAF from the Tactical Air Command - was great stuff.