Archie Forrest opens the door to his studio in the west end of Glasgow to announce he is in a bit of an "adrenaline frenzy".

It is the day before he must deliver the 60 new paintings he has been working on for the last 18 months for Ensemble, his fourth solo exhibition. The men in white coats are coming tomorrow morning to pick them up, and he knows he must put down his brushes. But he can't let go quite yet.

"I am not in the real world just now and am on a bit of a high, but come in and have a coffee and a wee Italian biscuit," he says, and for the next 90 minutes I am treated to a non-stop stream of consciousness from the man many regard as Scotland's leading post-Colourist painter.

His studio, acquired two years ago, is an entire suite of rooms in a stunning late Victorian apartment, and the original dark woodwork, stained glass, copper light fittings and north-facing light seem to suit the vibrant palette of the framed canvases stacked up all around us. Studio still life paintings of fruits and flowers, and scenes from recent travels in France and Portugal, are boldly described in bright reds, oranges, pinks, blues, greens and blacks.

Applied in Forrest's highly textured impressionistic style, they jump out like beacons as if to guide us through to the easel on which he is working in the light of the far window. The large canvas here, Devonshire Dream, is one of twin self-portraits for the exhibition. Forrest swiftly resumes his task of working walnut stained beeswax into the beautiful gesso and clay wooden frame to age it.

"I'm anal when it comes to frames, because they should be part of the picture," he says. "I am trying to extend the painting into the frame; a frame should not say, 'This is where the painting stops', and if someone says 'That is a nice frame' it is almost an insult because if it is separate from the painting, it is not doing what it is supposed to do."

Made to order by Mark Greer at Glasgow's WASPS studios, he praises the frames for being of "museum quality", by which he means they look like they belong in a gallery and "just behave themselves".

Devonshire Dreams presents the artist as a naked chef wearing nothing but a white toque and looking nervously out from behind a huge vase of white lilies.

"It's about anxiety, because I was thinking I had made a mistake in buying this flat, both financially and artistically," he says. "I was worried the light was not right and my paintings would not be right. So this painting conveys vulnerability, as cooking naked puts you in all sorts of danger, like burning your bum."

It sits in direct contrast to its sister painting, New Studio, New Hat, where the (fully-clothed) artist is wearing a black chef's toque and resembles a cross between a Portuguese fisherman and Hans Holbein. "It's a happier picture despite the black hat and I painted it before the other one as a celebration of moving in here."

Any previous angst appears to have settled into a positive joy in his surroundings, as evidenced in the fondness with which he describes his new body of work - and that northern light.

"North is best because it's the most consistent light and it does not move from left to right casting shadows across the face," he says. "Whenever you start in the morning, you can be sure it will be the same light at the end of the day." That said, he has invested in hi-tech pure daylight lamps to assist late-night working, as "painters in Glasgow have to wait until 1pm to get the best light".

He is fascinated by the effect of Scottish light on the colour notes taken in south France and Portugal. The brilliant red of a terracotta roof tile in south France might be made yet more red by the deep olive green of a plane tree or the azure blue of the sea, only for them all to jump off the page when the notebook comes back to Scotland; the pale green of a Portuguese fisherman's net might take its cue from the ultramarine sea water but reveals itself as jade back home; a Chinese garden might have pinks, violets and purples he only notices here because of our unique natural light.

"Painting is like making tunes, putting together notes, colours and materials," he says. "That is why the exhibition is called Ensemble, to give it a musical theme.

His work has often been compared to the Colourists such as Peploe and Cadell, and even Cezanne and Degas, the Scots' big influences and his own early heroes. Would he agree with that?

"My painting is the result of every painting I have ever been impressed by, but nobody likes being put in a box ," he answers. "As a child I spent my weekends at Kelvingrove Art Gallery looking at their frame of lilac coloured bits of wood, and wondered to myself, 'how can something with two strokes of paint describe something so well?'. So when I left Glasgow School Of Art I went to find that stuff, to find a way of looking and making notes and formalising ideas and adding my own ingredients.

"It is the technique of how I build paint in which I would be happy to be compared to the Colourists. I try to show the magic of paint and to be compared in that way with them would be a huge compliment."

How has his painting evolved over his 40-year career? "When I was younger I would look at more mature painters and could not believe how dextrous they were, so full of confidence. I think I have matured. I am starting to exude that thing I recognised in the older guys. There is less clumsiness, more confidence. A single brush stroke can make or break a painting. I love a bit of bravado. If you pull that off, the feeling is marvellous."

Yet Scotland remains largely uncharted territory, an omission he does not duck. He is quick to admit Scotland "has too many colours" for him and that he has so far postponed this ultimate challenge.

"I have not used Scotland the way I have used France," he says. "It has such an intense beauty I have always put it consciously on the back burner. Sometimes I do not think I am good enough for Scotland. So that is a project for the future."

Meanwhile, he is preparing for another psychological battle: letting go. "You get a bit bi-polar," he says. "I am on this high right now but when they are gone you start to come down. Come Sunday I will start to fall off the cliff. That is the thing about being a painter. You get fantastic phases and then the black hole.

"But there is nothing to compare to this. It is the best and the worst place to be and I wouldn't swap it for anything."

Archie Forrest's Ensemble is at the Scottish Gallery, 16 Dundas Street, Edinburgh, from November 3-26. www.scottish-gallery.co.uk