English term (edited): natural hair colour at the base

natural hair colour at the base=NATURAL BASE HAIR COLOUR
what the natural hair colour is at the base=WHAT A NATURAL BASE COLOUR IS
见链接
WHAT IS A NATURAL BASE COLOUR?
The level system is made up of 10 numbers which help determine the depth level of your natural hair. 1 is black, 5 is light brown, 6 is dark blonde, 7 is blonde and 10 is the lightest blonde. Your L’Oréal Professionnel colourist will analyse your hair and assess your natural base colour in-salon.
WHAT IS A BASE COLOUR?
The first number which appears on the colour chart is the base colour. This number indicates the base level of the shade you would like to achieve. It shows how light or dark the colour is.

--------------------------------------------------
Note added at 1天 23小时 (2021-01-01 13:14:46 GMT)
--------------------------------------------------

base color也可以翻译为固有色
https://tieba.baidu.com/p/5896955389?red_tag=0593249397

--------------------------------------------------
Note added at 1天 23小时 (2021-01-01 14:01:44 GMT)
--------------------------------------------------

base color还是“底色”最佳：
https://www.baidu.com/link?url=gLWLVbXGnFIzp4rWX3jkLbVb6aCPG...