If you selected a color for the sky in the 3D view pane or if the current mode is Aerial view, a sky image computed according to the hour of the day is used for the rendering of the sky. If you chose a texture for the sky, that texture will always be used in Virtual visit mode whatever the selected hour.
I guess what you see isn't completely gray but is the bottom part of the sky image. If you want a plain color in the photo you create, you should use a texture image containing only that color for the sky (it doesn't have to be big, a image with a few pixels will be enough) .
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Emmanuel Puybaret, Sweet Home 3D developer
