Beginner painting and adding mortar to brick building - Model Railroader Magazine

Depends on how different a red you want the building to be, and whether it is a completed kit or still in parts.   

It is possible to tone down the color of a raw plastic brick structure without new paint.  If the kit is unbuilt, the late Art Curren (the king of kitbashers) advised vigorous scrubbing with a kitchen cleanser like Ajax or Old Dutch Cleanser - something with a light abrasive in it.  This not only kills the shine but because the surface now reflects light differently, tends to change and lighten the darkness and tone of the color of the plastic.  This might be a different enough look to satisfy you. Depending on what details are on the building this may or may not be practical for a completed kit.

Another thing that can change the look is a coat or two of DullCote. It really does seem to change the color but again that is a function of how light is reflected or absorbed by the surface.   If you have clear "glass" windows however it can be a pain to mask every window.

Another benefit to DullCote is that the surfaces now have some "tooth" making the application of chalks or pastels with darker reds or browns effective, to be sealed with another coat of DullCote (and it may take another iteration before this "takes").

This is more iffy and unpredictable but a misting with a combination of isopropyl alcohol and india ink can make a remarkable change on both painted and unpainted structures (and rolling stock).  And this too gives a surface for chalks or pastels.

There are two or three schools on how best to create a lighter color for the mortar between bricks.  I have tried these and have failed and succeeded with them all!

The first is to paint with a very thin wash of diluted off-white acrylic or latex paint (some guys even use the liquid white shoe coloring meant to revive white canvas "tennis" shoes), and when that paint is almost dry, to wipe off the bricks so lightly that only the paint in the mortar lines remains.  I find that a cloth goes too deeply into the cracks.  A cloth wrapped around a flat block of wood or plastic touches only the top surface of the bricks.

The second is the same as above, but let the white wash dry completely and rub it off the surface of the bricks more vigorously again via mechanical means that involve a hard flat surface and cloth or paper towel, or even a pink pearl type eraser

The third is io paint the structure entirely with an off white, or a white with a touch of the brick color in it.  Let it dry completely.  Then "stamp pad" the brick color on - again perhaps with a cloth or sponge on a hard flat surface so that it does not reach down into the cracks.  Don't overdo the variety of brick colors - sometimes you do see old brick buildings with a very mottled look but in general if you stand far enough away from a brick building it looks quite uniform in shade and tone.

Other modelers have their own techniques. 

There are products, one called Roberts comes to mind, of specific premixed washes for mortar lines.  Looking at a variety of brick buildings one sees quite a bit of variety - sometimes the mortar lines are almost stark white, or off white, but often they are almost the same color as the brick.  It seems like the stark white are more likely to be residential homes where that "look" is part of the intent.

Dave Nelson