You just need to specify the color along with any other tags. So in the gallery you'd have something that looks like this (ignore the * character - those are just there so the forum will display what I type inside the square brackets rather than interpreting them and formating the text accordingly)
[*url="http//:
www.myblog.com/blogpost.htm"][*color=blue]click here for my blog post[/color][/url]
You always need to close a tag so as just as you'd type "[/url]" to finish the web address, you also type "[/color]" to finish the change of color. Those should be "nested", IYSWIM.
You can also use [*b] (with [*/b] to close) to make text bold. So for the example above to be bold as well as blue, that would be:
[*url="http//:
www.myblog.com/blogpost.htm"][*color=blue][*b]click here for my blog post[/b][/color][/url]
HTH!