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what is the structure of double salt and complex salt,and the properties between double salt and complex salt?why double salt and complex salt have different color?

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Double Salts:   contain more than one anion or cation.  They form when more than one salt is dissolved in a liquid and when together they crystallize in a regular pattern.  Alum is an example of a double salt.  Alum contains aluminum and sulphate ions.  Another example would be potassium sodium tartrate.  Epsom sals are an example of a double salt.  A double salt is a salt that crystallizes as a single substance but ionizes as two distinct salts when dissolved, as carnallite, KMgCl3·6H2O

Complex Salts:  A class of salts in which there are no detectable quantities of each of the metal ions existing in solution; an example is K3Fe(CN)6, which in solution has K+ but no Fe3+ because Fe is strongly bound in the complex ion, Fe(CN)63-.

Many of the transition metals form coordination compounds or complex salts. You can prepare a complex salt of copper:   Cu(SO4).5H2O + 4NH4OH ----> Cu(NH3)4SO4. H2O + 8H2O.

A coordination compound is composed of one or more complex structural units, each of which has a central atom bound directly to a surrounding set of groups called ligands. Coordination compounds are substances with characteristic chemical structures in which a central metal atom is surrounded by nonmetal atoms (or groups of atoms), called ligands, joined to it by chemical bonds.

Link to structures:  http://en.wikipedia.org/wiki/Complex_(chemistry)

Transition elements include, first series - Cr, Mn, Fe, Co, Ni, Cu and Zn, second series - Mo and Ag, and third series - Pt, Au and Hg. Transition elements are hard, dense metals with high melting points and boiling points, form colored ions and compounds and have more than one valence (oxidation number), such as copper (Cu1+ and Cu2+) and chromium (Cr2+ and Cr3+). The ion Cr2+ is a strong reducing agent and forms blue salts in solution. Cr3+ salts are green in solution. CrO42- salts are yellow and Cr2O72- is a strong oxidizing agent with orange salts, such as K2Cr2O7.

This link explains more about color:  http://en.wikipedia.org/wiki/Colors_of_chemicals