Clear window glass and a deep blue church window are made of almost the same thing: mostly silica (sand), with soda and lime added so it melts more easily. The difference is a tiny amount of something else. This guide explains how glass gets its colour, which ingredients glassmakers use, and why coloured glass is such a good model for PaneSlide's puzzles.
Colour is what is left over
White light, such as daylight, is a mixture of many colours. When light passes through coloured glass, the glass absorbs some colours and lets the rest through. The colour you see is what is left.
- Blue glass absorbs much of the red and yellow light and lets blue through.
- Yellow glass absorbs blue light and lets red and green through, which your eye sees as yellow.
- Red glass absorbs most of everything except red.
Stack two pieces and each takes away its share, so less light gets through and the colour changes. This is called subtractive mixing, and it is exactly how PaneSlide mixes its panes. The colour mixing guide explains it in more detail.
The ingredients behind the colours
Most colour in glass comes from small amounts of metal compounds dissolved in the melt. A few of the classic ones:
| Ingredient | Typical colour |
|---|---|
| Cobalt | Deep blue, even in very small amounts |
| Copper | Turquoise or blue-green; under some conditions, red |
| Iron | Green or blue-green; with sulphur and carbon, amber or brown |
| Manganese | Purple (amethyst); in tiny amounts it hides iron's green tint |
| Chromium | Strong green |
| Gold | Ruby red, or pink "cranberry" glass in lower amounts |
| Selenium | Pink to red; with cadmium sulphide, a brilliant red |
| Cadmium sulphide | Deep yellow |
| Uranium | Yellow-green that glows under ultraviolet light |
The exact colour depends on the amount, the rest of the glass recipe, and how the glass is heated and cooled. The same metal can give different colours in different conditions.
Why window glass looks green at the edge
Look at the edge of a thick piece of ordinary window glass and you will often see a green tint. That comes from small traces of iron, which are hard to remove from sand. Through the face of the pane, the glass is thin and the tint is too faint to notice. Along the edge you look through much more glass, so the green builds up.
Glassmakers have long added manganese or selenium in tiny amounts to cancel the green. Manganese was sometimes called "glassmaker's soap" for this reason.
Colour from tiny particles
Some of the most striking colours come not from dissolved metal but from extremely small particles suspended in the glass. Gold ruby glass is a famous example. Very small amounts of gold, around ten parts per million, give a rich red. The colour depends on the size of the particles, so the glassmaker controls it by heating the glass again after shaping. This is sometimes called "striking" the colour.
The most famous ancient example is the Lycurgus Cup, a 4th-century Roman glass cup in the British Museum. It looks green when lit from the front and red when lit from behind. Scientists have found tiny particles of gold and silver in the glass that cause this effect. The stained glass history guide tells more of the story.
Flashed and painted glass
Some colours, especially deep reds, are so strong that a full-thickness sheet would look almost black. Medieval glassmakers solved this with flashed glass: a thin layer of coloured glass fused onto a clear base. Artists could even grind or etch away parts of the coloured layer to make patterns.
Glass can also be painted with dark paint made from metal oxides and powdered glass, then fired so the paint fuses to the surface. This is how faces, folds of clothing and fine lines are added to stained-glass windows.
How this relates to PaneSlide
PaneSlide simplifies real glass to three ideal filters:
- Cyan glass takes away red light.
- Magenta glass takes away green light.
- Yellow glass takes away blue light.
Each pane takes away exactly one part of the light, so every overlap is predictable: two panes leave blue, green or red, and three leave black. Real glass is never quite this tidy, but the principle is the same one that colours a cathedral window. When a target picture shows green, it is telling you that cyan and yellow glass share that cell, just as the glow through a real window tells you which glass the light passed through.
Tip. If you like the look of stained glass, try the Midnight board skin in the Shop. Dark frames make the colours stand out, a bit like lead lines in a real window.
Sources
- Wikipedia, "Glass coloring and color marking" (metal colourants, gold ruby, selenium, uranium, iron tint, manganese as decolouriser), checked 29 October 2026.
- Wikipedia, "Lycurgus Cup" (4th-century Roman cage cup, British Museum, gold and silver nanoparticles), checked 29 October 2026.