There is quite a lot of information about how to set the insert mode cursor color in the vim documentation
Here is an example from the linked documentation:
set guicursor=n-v-c:block-Cursor (was enough for me)
highlight Cursor guifg=white guibg=black
highlight iCursor guifg=white guibg=steelblue
set guicursor=n-v-c:block-Cursor
set guicursor+=i:ver100-iCursor
set guicursor+=n-v-c:blinkon0
set guicursor+=i:blinkwait10