Кто сможет ответить,почему на г.р.з. 64 черные буквы на желтом фоне,а у 63 и 65 белые буквы на черном фоне?

Желтые номера - более современные.

The black/white system ended in 1973...call it transition phase.
(Luxbg. City had ten O305 (153-162) with white/black plates!)

64 was the only one of this series (62-73) which got delivered with the yellow plates.

Btw., those buses got painted by schoolchildren because they were a gift to the Islands.