[jolly]

Why are lemons Yellow and oranges orange, what dictates a fruits colour?

[Karen W.]

 What good Questions!!!!

[paul.fr]

Lemons are yellow because their skins absorb all wavelengths of visible light except for the wavelengths corresponding to the color we know as yellow.

[Karen W.]

Cool Thanks Paul!

[paul.fr]

the chemicals in the skin/rind may also have an influence on the colour.

[_Stefan_]

Yes, it's pigmentation, not structural colour.

[jolly]

Yes fine but why? what is it that made the lemon gain a yellow and not a red or pink colour?

[_Stefan_]

Because their genes cause them to produce yellow pigments but not red.

http://en.wikipedia.org/wiki/Chromatophore

http://en.wikipedia.org/wiki/Biological_pigment

[jolly]

Because their genes cause them to produce yellow pigments but not red.

http://en.wikipedia.org/wiki/Chromatophore

http://en.wikipedia.org/wiki/Biological_pigment

Is that an answer?

[_Stefan_]

What kind of answer do you want? Have you tried researching plant biology?

[WylieE]

Hi Jolly,
 Good question!  I'll give a quick answer now with a bit that I know and try to do some more research and get you a better answer.  So really it depends on how the fruit is dispersed.  So in this case fruit color is similar to flower color.  Plants attract the best pollinators by 'advertising' with colors that the best pollinators like.  It is similar for fruits, although with all the domestication we have done of fruits it isn't that straightforward anymore.  However, fruits that are typically eaten by birds and mammals tend to have bright flashy colors- reds, yellows, orange, things that stand out dramatically against the green forest.  So these fruits seem to say "here I am pick me and disperse the seeds."  Birds and mammals are a great choice for a plant because they will spread the seeds far.
  Some fruits stay green because the plant actually needs to do photosynthesis in the fruit while the seeds are growing.   The whole process of how a fruit changes colors is very fascinating.