Hohenems is right, what is missing in your proposal is the verb: "is (yellow)" and "est (jaune)".
The person is inquiring about the color of the lemon (could be green...).
A sentence starting with "est-ce que le citron jaune" would need to be finished with something like "est-ce que le citron jaune... est meilleur que le vert ?" (is the yellow lemon... better than the green one?)
In other words, the first "est" in "est-ce que" has no role in terms of verb. It is just one of the ways to introduce a question in French.
If you need more: http://french.about.com/od/grammar/a/questions.htm
In French, there are several ways of asking questions, from formal to standard to relaxed registers of speech:
- is the lemon yellow
= le citron est-il jaune ? (formal), with main subject repeated as a pronoun in inversion Verb-Subject.
= est-ce que le citron est jaune ? (standard)
= le citron est jaune ? = the lemon is yellow? (relaxed)