Tiles obtained from the same source in the same colour and finish should match exactly, but unfortunately will not be identical, due to ingredients or temperatures being slightly different during manufacture. The old tiles will also have been subjected to many years of weathering from acid rain and other atmospheric pollutants.

Identification
By looking at the area of the tile covered by the head lap, it should be possible to identify its colour and finish. Turn the tile over to identify the manufacturer by a mark, name or registered design number. This number will not provide the colour reference but should give the design and approximate age of the tile. Most modern tiles have ink jet markings that give batch information to identify the place and date of manufacture. From this it may be possible to obtain secondhand tiles from a roofing or salvage merchant. Secondhand tiles will have a weathered appearance but may still not look right as they may have been positioned on a different facing roof slope, with more or less moss growth.

Cleaning
Another option may be to power wash the roof to remove moss, lichen and algae. However, this may result in a patchy appearance, especially if the tiles were granular faced. Where lichen has grown, levels of acidity will have been greater, causing the cement slurry to erode quicker, resulting in a greater surface granule loss.
      Power washing can also force water and debris in joints and interlocks further under the tiles, or push it onto the underlay where it will become a problem at a later date. At worst, water forced through laps and interlocks will run down the underlay and may show itself inside the roof where there are holes in the underlay or where there is a lack of tilt fillet behind the fascia board.
      Clambering over the roof, even with roof ladders and crawl boards, may result in tile breakage or the disturbance of mortar bedding, which may not be immediately visible. Debris washed off the roof has been known to block rain water pipes. Not only will the gutter need cleaning, but also the down pipes and gullies. The only safe and thorough way to clean roof tiles is to remove them, clean them and re-fix them (only if they are not damaged).
      If you suspect the tiles are made from a material containing asbestos it may be safer to leave well alone and seek advice from an asbestos removal expert.

Prevention
At this stage it may be a good idea to treat the roof to prevent the return of moss and lichen. This can be done in one of several ways; position a large copper, zinc or lead flashing along all ridges and hips and after a shower of rain a deposit of copper, zinc or lead oxide will flow down the roof and poison moss or lichen. If the flashing is not large enough only the area next to the ridge or hip will be protected. This option may not be advisable if water run off is collected for watering plants or domestic use.
     

Coating
Some clients are tempted to ask for the roof to be painted with a pigmented coating to restore the original colour. Whilst this may make the new and old tiles look the same, it can often look patchy due to the lack of quality control during the application, which is difficult to achieve in the open air. Like all coatings, they do not last forever. They will be affected by the elements and will start to break down within a few years and may look worse than before. Also, a build-up of the coating may congeal in the interlocks and laps and restricts the free draining of the roof system, especially if they have not been thoroughly cleaned.

Promoting growth
An alternative is to do the opposite; treating the new section of roof with nutrient should encourage the growth of moss and lichen to match the old section of roof. The nutrient could be skimmed milk/yogurt (natural not fruit), liquid tomato fertilizer or liquid cow manure. The liquid cow manure option is cheap but may turn the whole roof brown for a short period of time and have a rather strong smell. The application should be sufficient to cover the roof but not congeal in the joints and interlocks where it may do more harm than good.

Grouping
The best solution may be to strip a section of the existing roof and use the good salvaged tiles to cover the remainder of one elevation and put all the new tiles on one or more complete elevations to maintain the new or old appearance of the roof. This also has the benefit of allowing part of the roof to maintain the tile manufacturer’s guarantee, as laying new tiles with old tiles may invalidate a guarantee, especially if the design of the tile has been improved and are not backwards compatible.
With clay plain tiles, thorough mixing can produce a pleasing random, mottled appearance, but to achieve this successfully the tiles all need to be of the same type, preferably from the same manufacturer. Mixing double camber handmade tiles with single camber machine made tiles can look a mess.
Whichever method is decided upon, within about ten years all sections of the roof should be almost indistinguishable due to the gradual deposit and effect of atmospheric pollution.