True Type Fonts on PNG-Pictures cause more colours
If you create PNG pictures and use True Type Fonts be aware that the color index palette might increase significantly. This happens even if you define only one color (e.g. with imagettftext) for the writing. The new colours are usually less intense variants of the defined colour. If a less intense coulour is already set, then this index is used instead.
For example if you chose a grey text colour like RGB 210/210/210 then partly 230/230/230 will be used. If this colour is already defined, that index will be used. Otherwise a new colour will be created automatically.