There comes a point in mid-October, usually after the first genuinely cold night, when a British wood seems to hold one colour and one colour only. You walk in under beech and sycamore and the whole floor is rust, ochre and quiet brown. It is a short window. A week of wind and rain, and those leaves will be a sodden mat underfoot rather than something you can actually paint. So it pays to treat the leaf fall as a gathering season, not just a pretty backdrop, and to come home with enough evidence to build a palette you can use all winter.

Collecting leaves that still have something to say

Pick leaves that have fallen recently and are dry, flat and uncurled. Anything damp will go mouldy in a book and anything brittle will crumble the moment you press it. Aim for six to ten good specimens rather than thirty mediocre ones, and choose them for variety of colour rather than variety of species.

Some reliable performers:

• Field maple — a clear gold that often carries a pink flush along the edges.
• Horse chestnut — deep amber fading to conker brown at the tips.
• Beech — pale orange-bronze, and it will hold on the tree until spring, so you can compare fresh and weathered colour.
• Oak — a soft, tawny russet with a green ghost still in the veins.
• Sycamore — clean yellow-brown, generous in size, easy to paint.
• Wild cherry — claret and crimson, useful when you need a warm red that is not orange.

Press your haul between sheets of kitchen paper under a couple of heavy books for three to five days. Write the date and place on a strip of masking tape on the back of each one. If a leaf is too brittle to press, keep it loose in a rigid box and work from it quickly.

Reading the colour before you mix it

Hold each leaf against a sheet of white cartridge paper in daylight. Never judge colour under a warm indoor bulb — you will mix everything too yellow. Ask three questions. Which way does the hue lean, towards red or towards yellow? How light or dark is it? And how greyed has it become, now that the green has gone?

Then turn it over. Undersides often tell a different story: beech undersides are a pale milk-chocolate, oak keeps a faint green cast underneath long after the top has turned. Look at the veins and the stem too, because they hold their green longest, which is why one leaf can contain four separate colours.

Keep your colour vocabulary sharp while you do this. Ochre, tawny, umber, brick, khaki, burnt orange. Write the words in the margin. A descriptive word you can act on later is worth more than a vague memory of "brownish".

Turning leaves into pigment notes

Keep a small box for the season: yellow ochre, raw sienna, burnt sienna, burnt umber, one warm red, ultramarine and something transparent and glowing such as quinacridone gold. From that set you can reach most of what autumn offers.

• Beech bronze — burnt sienna with a touch of quinacridone gold.
• Oak russet — burnt umber and burnt sienna in roughly equal parts, well diluted.
• Horse chestnut amber — raw sienna with a whisper of transparent orange.
• Sycamore gold — yellow ochre with a hint of raw umber to take the brightness down.
• Leaf-litter shadow — burnt umber with ultramarine at about three to one, which gives you a cool, quiet dark brown.
• Green holding on in a vein — sap green with a touch of alizarin to grey it off.

Test each mix as a small square, and test it twice: once wet and once dry. Watercolour dries noticeably lighter, so compare your dried swatch against the leaf, never the wet one.

Building the palette page in your sketchbook

Give one page, landscape orientation, to the season's palette rather than scattering swatches through the book where you will never find them again. Divide it into a grid of eight to twelve squares. In each square put a wet swipe, a dry patch, and one line naming the mix.

Beside each square, tape down a tiny fragment of the actual leaf with a scrap of washi tape. In February, when the colours have gone from the world entirely, you will be able to hold your paint against the real thing. Add a vertical strip down one side showing your best three mixes at full strength, half strength and a pale wash. Label everything with the date and the place you gathered. Next year's page will not be identical, and that difference is worth recording.

Sketching outdoors while the light holds

Late October light is gone by half past three, so work small and work fast. Draw the silhouette and a few veins with a fine waterproof pen, then lay a wash over the top. Resist outlining every serration; a suggestion of the edge reads better than an accurate wobble.

Sketch leaves where they fall — a drift against a root, a scatter along a kerb — rather than pinning one specimen out on a table. You learn far more about colour relationships from a group than from an isolated leaf. Travel light: a six-pan tin, a waterbrush kept in an inside pocket so the water does not freeze your fingers, and a pocket sketchbook. Note the weather on the page, "damp, low sun" or "bright after rain", because those two words will explain your colour choices when you look back at the page in January.