Hi Ryan,
This is in our wheelhouse as we are also masonry restoration contractors.  There are three possible scenarios to check:  Look all around the house and see if there are any patterns under overhangs, on the north side only.  Look for distinctive breaks like you have in your one shot which would imply a poor color match...see below.
1.  Is the mortar wet/new?  Areas in the shade can cure slower.  Also discoloration can be biological growth, but not in this case on a north side of a house where the mortar dries slowly after a rain
2.  Not matching the mortar color exactly.  If you don't weigh the colorant and mix precisely you will get different shades.  It is easier to darken than lighten.  There is a product called liquid dirt by Edison Coatings that you can get in different shades ( no, I am not kidding).  It helps the new mortar look weathered until the natural weathering occurs.  This can be achieved with other means by someone who is trained in this kind of thing.  We have used a little diluted muratic, not recommending with natural stone to lighten darker mortar but you need to know what you are doing.  Literally you are creating lightness through an acid burn.  Again,  be trained for that.  
3. Sometimes if you get a rain before mortar sets, it can bleach which might explain this.  My best guess is an incorrect color match.

I am not sure if hydrogen peroxide will bleach this.  Use a cleaning product like Peroxy or H2Orange by Envirox rather than other forms so it won't be too strong.  
If you try anything, do it in a concealed place with the customers permission.  I hope this helps.


President
Mobile: 309.251.3192
 
 


-------- Original Message --------
Subject: Side Of House
From: Ryan Tonnemacher <ryan@beyondstonesolutions.com>
Date: Mon, June 12, 2017 1:18 pm
To: "sccpartners@stoneandtilepros.simplelists.com"
<sccpartners@stoneandtilepros.simplelists.com>

We are wondering if anybody has any ideas how to make this mortar in between the stone uniformly light. As you can see in the pictures most of the mortar has turned to be much darker than intended. In one of the pictures below you can see the lighter mortar and darker mortar, we are trying to achieve the lighter color mortar throughout the entire area.
 
 
Thank you, we appreciate your business!
 
Ryan Tonnemacher
General Manager
(602) 993-9800 Office
(480) 459-5773 Fax
2039 E. Cedar St. #104
Tempe, AZ 85281
www.BeyondStoneSolutions.com
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INSTALLATION – RESTORATION – MAINTENANCE
 
 
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