Four Colours Solver

Four Colours Solver — Step by Step

Drag across squares to join them into regions, then pick a colour and tap regions to give their starting colours. Press Solve. Or play a generated puzzle and send it here with one click.

Four Colours Solver: rules, how to use it, tips

Colour every region with one of four colours so that regions sharing a border never match.

The rules

Every region is coloured red, yellow, green or blue.
Two regions that share a border — even a short one — must have different colours.
Regions that only touch at a corner may have the same colour.
Some regions start coloured, and there is exactly one way to complete the map.

Using the solver

Choose the size, draw the regions and add the given colours (or press Sample).
Press Solve and step through: regions forced by their neighbours, narrowing down, and what-if chains.

Tips

Three around one. If a region’s neighbours already use three different colours, it must be the fourth.
Narrow it down. Cross off the colours a region’s neighbours already use; keep track of what is left.
Pairs. Two neighbouring regions that can each only be red or blue use up both colours — any region touching both can be neither.
What if. When stuck, try a colour in a region with two options and follow what it forces. If something ends up surrounded by all four colours, the try was wrong.

The full guide, with a worked example and common mistakes: How to play Four Colours.

Questions

Why four colours?

Four is always enough for any flat map — that is the Four Colour Theorem. Three is not always enough.

What do the shapes mean?

Each colour has a shape (● red, ▲ yellow, ■ green, ◆ blue) so you can tell them apart without colour.

Is there always one answer?

Yes — the starting colours are chosen so there is exactly one way to finish.

How do I solve one from a book?

Open the Four Colours Solver, draw the regions, add the given colours and press Solve.