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From "The Chemistry of the Liquid Alkali Metals", C.C. Addison, Wiley 1984.
When free from surface contamination, the metals Li, Na, K and Rb are silver-bright and lustrous. Caesium, however, is pale gold in colour in both solid and liquid states; coupled with the low density, and the low viscosity of the liquid, it surely must be the most attractive element in the periodic table. Earlier workers attributed the colour to the presence of trace amounts of oxide impurity, and certainly the gold colour does darken on the addition of oxygen. However, intensive purification of the metal by all available methods in the writer's laboratory over several years has not succeeded in removing the colour, and we have no doubt that caesium metal is indeed pale golden in colour.
Reference goes on to explain the color in terms of the band structure of the metals, the salient point being that for higher atomic numbers, overlap of the s with the d orbitals makes a more significant contribution to the band structure, generally lowering the energy required for exitation of conduction band electrons, until in the case of cesium there is significant absorbtion in the blue region of the spectrum producing the golden color.