Wien's Law tells us that objects of different temperature emit spectra that peak at different wavelengths.
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Hotter objects emit most of their radiation at shorter wavelengths; hence they will appear to be bluer .
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Cooler objects emit most of their radiation at longer wavelengths; hence they will appear to be redder.
Furthermore, at any wavelength, a hotter object radiates more (is more luminous) than a cooler one.
[Source:
http://www.infraredtraininginstitute.com/images/stories/iti/wiensdisplacementlaw.png]
[Source: http://www.tankonyvtar.hu/hu/tartalom/tamop412A/2010-0010_02_Applied_Remote_Sensing/4457/objects/il_0_mob_11928/kep_02_02_01_05_angol.jpg]