I think Adam's solution is probably the best fit for your needs.
However, if you need a bit more flexibility (eg if adding BLUE to ALERTCOLORS causes blue dots to show on your "Current non-green systems" page, as if they were active faults), there may be (ie, I have not tested) another solution. You can use the "xymond_channel" binary to tap into the "stachg" (status change) channel, filtering for any event that changes colour from red to blue or from blue to red (or any other transition you're interested in). The following command waits for such events, and for each it spits out the associated status message that you might use in an email message:
$ sudo -u xymon xymoncmd xymond_channel --channel=stachg --metafilter='\|(blue\|red|red\|blue)\|' cat
Here I'm using "cat" to send the message stream into, just for demonsting. You would want to replace "cat" with a script that extracts the text between lines matching regex /^@@/ and inject them into an email program.
There might be a way to redirect this output back into to Xymon on the "page" channel, which then allows your existing alerts.cfg configuration to be used for managing your emails in the same way you would for red alerts, etc. However I suspect there will be limited features (eg there isn't a DELAYBLUE option) or work-arounds required (eg specifying a colour of "!red,!yellow,!purple" to match "blue" events?)
J