I've never dyed my hair in my whole life, since I like my black hair. I've never dyed my hair in my whole life, since I like my black hair.

Adding "whole" to describe "life" helps emphasize that you have never dyed your hair at all.

I've never dyed my hair in my life, since I like my black hair. I've never dyed my hair in my life, since I like my black hair.

I'’ve never dyed my hair in my life sincbecause I like my black hair. I’ve never dyed my hair because I like my black hair.

'My' hair, in 'my' life, 'my' black hair. It just gets redundant and doesn't feel as smooth in a normal conversation..

I've never dyed my hair in my life since I like my (natural) black hair. I've never dyed my hair since I like my (natural) black hair.

“never” already means at no time in your life.

Black hair is generconventional in Japan, and I think my natural hair color suits me better. Black hair is conventional in Japan, and I think my natural hair color suits me better.

"conventional" seems to be a better fit for what you wanted to say.

Black hair is generalcommon in Japan, and I think my natural hair color suits me better. Black hair is common in Japan, and I think my natural hair color suits me better.

Things that exist in high proportions are "common"

Black hair is generalcommon in Japan, and I think my natural hair color suits me better. Black hair is common in Japan, and I think my natural hair color suits me better.

"common" is a more natural word choice here.

Black hair is generalvery common in Japan, and I think my natural hair color suits me better. Black hair is very common in Japan, and I think my natural hair color suits me better.

Black hair is generalcommon in Japan, and I think my natural hair colorit suits me bestter. Black hair is common in Japan, and I think it suits me best.

If I dyed my hair once, I would need to keep dyeing it like a subscription, and it's. It would be very troublesome and costly. If I dye my hair once, I would need to keep dyeing it like a subscription. It would be very troublesome and costly.

If I dyed my hair once, I would need to keep dyeing it like a subscription, and it'swhich would be very troublesome and costly. If I dyed my hair once, I would need to keep dyeing it like a subscription, which would be very troublesome and costly.

Two possible sentence structures here: "If I dye... I need to..." "If I dyed... I would need to..." Good job spelling "dyeing" correctly, I have found most native speakers get confused about this word

If Iyou dyed my your hair once, Iyou need to keep dyeing it, like a subscription, and it'which is very troublesome and costly. If you dye your hair once, you need to keep dyeing it, like a subscription, which is very troublesome and costly.

Because this is a general statement it's usually expressed with the word "you" instead of "I"

If I dyed my hair once, I would need to keep dyeing it like a subscription, and it'swhich would be very troublesome and costly. If I dyed my hair once, I would need to keep dyeing it like a subscription, which would be very troublesome and costly.

'If I dyed my hair once' is a hypothetical so you would also need the rest of the sentence to be in such a tone. To keep it same you would use 'would'.

If I dyed my hair once, I needwould have to to keep dyeoing it like a subscription, and it's very troublesome and costly. If I dyed my hair once, I would have to to keep doing it like a subscription, and it's very troublesome and costly.