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Were abouts is the file that stores this?

Hello There, Guest! (Login — Register)

Also when i am editing it how can i add color and bold tags?

Admin CP --> Templates --> Modify/Delete --> *expand your template set* --> header_welcomeblock_guest.

If you want to actually change the text, you need to edit inc/languages/english/global.lang.php (don't use Notepad)
Find:

$l['welcome_guest'] = "Hello There, Guest!";

and edit it.