Consistency around SvxColorItem in svx/svx3ditems.hxx

...similar to what has been done for svx/sdtmfitm.hxx in
68969cc61adecac481ae9656978ef952f435b310 "Consistency around SdrMetricItem."

Change-Id: If26ab3229871d2d6a7e4e7e8f79f4cb927b96930