How can I only change comment color?

[LvChengbin]

I want to make comment color with more bright, what config items do I need to add in .vimrc file?

[matass]

highlight Comment guifg=#ffffff

[Luxed]

I want to make comment color with more bright, what config items do I need to add in .vimrc file?

You can look at :h :highlight (also found here) for information on how to change highlight group colors.
Then, there is also a small explanation on how to have an automatic setup that will only the ayu theme on my maintained fork. Although I only explain how to use my function to modify the colors, you can also use the :highlight command in there.
The most important thing to remember is that this modification needs to be done after you have loaded your colorscheme with colorscheme ayu.