Currently mark character is bright red as in below screenshot. How to change it?
By default, the vim-signature plugin uses the
SignatureMarkText highlighting group for its mark signs.
It's better to set the colour of this in your
vimrc rather than editing the plugin's code (which could cause problems if/when you want to update the plugin).
Add the following line to your
highlight SignatureMarkText guifg=White ctermfg=White
(You can also run this command from command-line mode to apply the change without restarting Vim.)
Note that the plugin uses the same mechanism for setting the colour, the
CheckAndSetHL() function (for which you altered the parameters in your answer) builds essentially the same command as the above.