Transition element exhibit colour due to d-d transition. d-d transition is possible only when d subshell have unpaired electron. Cu+ has completely filled orbital: [Noble gas] 3d10, where as Cu2+ has partially filled orbital : [Noble gas]3d9. Hence Cu+ is colourless but Cu2+ is coloured.
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70.Which of the following do you expect to be coloured and why? Cr+, Cu+?