graph items that have been created with older zabbix versions have problem with colour setting - in graphs everything is shown ok, correct colours are used, but in item selector a warning is printed :
"Warning. Field [color] is not color"
and the colour is set to "FFFFFF".

it seems that colours were not migrated from some older version (the exact breakage was not observed as graphs themselves continued to work ok), but parsing still understands old colour notation.

there should be an sql update which could recognise all older preset colours and convert those to the new notation.