Roads on the Landranger 1:50000 maps that you'll generally be using come in four colours: Red, Brown, Yellow and White. The other one, not used so often, is blue for motorways.
Each junction on the route is described in terms of the colours of the roads involved. The entry road is given first and the exit road last. So the instruction would be something like:
BRR RRY YYY YYY YYY YYY YYY YYBB BBY
You can tell that you're starting on a brown road which finishes at a red road. You'll have to decide whether you go left or right on the red road, but the next junction must be a turning off the red road onto a yellow road. Then you go through 5 YYY junctions before coming to a crossroads with a brown road. And so on...
Coloured Roads Only: Often abbreviated with the acronym CRO. If this appears on the route instruction it means you can ignore any white roads on the map when plotting your route. When the organiser intends you to use white roads you might see CAR - "Consider All Roads" or MUW - "May Use Whites", but usually the absence of CRO on the card is enough.
When white roads appear on tulips or herringbones they are usually drawn with a broken line, while coloured roads are drawn with a solid line.