My neighbour has recently had some work done to her roof. The old mortar was red and the new mortar is grey (uncoloured). So it looks a little silly with both colours present.
She's not very happy about this and when she complained they told her that colouring the mortar would make it weaker and more likely to crumble.
Whilst that sounds feasible it also sounds like an excuse. Surely the strength of the mortar is down to the ratio & quality of the mix, isn't it?
(The mortar at the front of the house is red, the replaced mortar at the side is not)