Reference:Background

A background color can be specified if desired. Any ray that does not hit an object will be colored with this color. The default background is black. The syntax for background is:

BACKGROUND: 
 background {COLOR}

Note: As of version 3.7 some changes have been made to the way alpha is handled when +ua is activated.

• In previous versions, specifying a background with the background keyword would by default supply a background with transmit set to 1.0 (i.e. fully transparent provided that +ua is being used). This is no longer the case. While the default background is transparent, any background specified in a scene file (unless 3.6 or earlier compatibility is being used) will now be opaque unless transmit is explicitly given. In other words, use rgbft<> rather than rgb<> in the background statement if you want the old behavior.
• The way that objects are blended with the background has changed. Previously the color of the background was not taken into account when calculating effects of transmission through translucent objects when +ua is in effect (i.e. where the background could otherwise have been seen through the object). Now, however, the background color is taken into account, even if it is not otherwise visible. Blending is performed in the same way regardless of the presence of background transparency.

Note: When using Output_Alpha=on or +ua with legacy scenes (the #version directive set to less than 3.7) the background will be suppressed, except in reflections.