colour is perception
the eye sees colour in different ways
colour is mixed differently dependant on if it is reflected or emitted
the primary colours for reflected light and red, blue and yellow where the primary colours for emitted light are red, blue and green
the combination of all colours is black if reflected and white if emitted

a leaf is a leaf, green is only the most prevalent reflected colour, in a dark room it is no longer green due to the lack of light to reflect the colour green but is still a leaf

stars emit the entire light spectrum but in varying degrees of intensity, that's why they appear to be different colours

the sun is called a yellow star because it emits more of the yellow spectrum than any other but still emits every colour (as shown in rainbows)

i could go on (and on, and on) but i won't... i have to go to work