Lets understand this by an example-
There are parts in a candle flame called 'blue zone' and 'non luminous zone'. In both the zones the flame appears blue. This is because these zones get a sufficient amount of oxygen i.e., supporter of combustion. 
In the same way the bonfire appeared to be blue, when it was surrounded by trees, which constantly give out oxygen. So the fire has the sufficient supply of oxygen and thus appears blue in colour.

can the red color of the flame and green color from behind, R+G => blue color ? combine to give blu e ?
I don't know abt that.........
oxygen is available every where in an open area.. so your answer does not seem to be appropriate
But when trees are around the oxygen levels in the air rise.........
trees are in the background. bon fire is not in the middle of trees.

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This is due to color combinations and color mixing.  The color of the flame (or the hot gases rising from the fire) and the color of the trees (yellow leaves/green leaves) have combined to give the blue color.

You that white light is a combination of many colors as shown in its spectrum.  Main colors are red, green and blue.  These are more or less the fundamental colors. 

These are used in generating other colors, in general. An object appears red, when it absorbs all other colors and reflects or emits only red color. Thus when paints of different colors are mixed, you get a different color.  This is used in real life.

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