THE exhibition of more than 100 works of the Glasgow artist Donald Bain which opens on Saturday offers as many mysteries as it solves about one of the most fascinating characters from the colourist school. Curated by Bill Hardie, who first met Bain in the late sixties, it demonstrates not just how many different stylistic avenues Bain explored, but for how short a period he loitered in any one of them.

Your appetite is whetted by some particularly vibrant work only to find that it was one of only two or three such canvases completed. Sometime, indeed, just one of

a kind, before Bain embarked on a fresh journey of discovery.

Some of the paintings completed within weeks of each other not only don't look contemporaneous, but you can hardly believe they came from the same hand. It wasn't so much a low boredom threshold as that traditional artistic anxiety about staying one step ahead of the bank manager, Hardie supposes.

Bain was born in Kilmacolm and lived for a while in England. But he came home to Glasgow at a time, says Hardie, ''which propelled him into the perfect milieu''. He was welcomed into the lively salon run by J D Fergusson and his wife Margaret Morris, whose house was a magnet for some of the most creative talents in the city at the time.

And it was Fergusson who arranged for Bain to try his luck as an artist in Paris immediately post-war, a city where he could make extensive use of Fergusson's contacts. Throughout that period, and when he finally came home, Bain and Fergusson kept up a regular correspondence and, thanks to the Fergusson Gallery in Perth, Hardie has been able to include some fascinating excerpts from the correspondence in the catalogue.

Although Fergusson's epistles to Bain were destroyed on the instructions of Eunice, his widow, the letters from Bain demonstrate not just how close the two men were, but also how generous the Fergussons were when Bain had too much month left over at the end of his money . . . a not infrequent occurrence.

One of the ironies, says Hardie, is that while Bain is often thought of as a disciple of Fergusson's, none of his early work demonstrates it although he exhibited alongside Fergusson and other members of the New Scottish Group in 1943, the year after it was founded with Fergusson as president. Says Hardie: ''It was, oddly, only when he began to paint in France that you could see the palette was Fergusson's.'' Judging by the letters, if Donald Bain lacked confidence in his powers of fiscal survival, that lack of confidence certainly didn't extend to his own work. He clearly had a good conceit of his self-taught abilities, but one with which you can hardly argue faced with the richness of the legacy on view at this retrospective. He was also quite capable of being underwhelmed by some of those international painters feted by others in the French capital or even his own Scottish colleagues:

''Wed. 27th Nov 1946. Yesterday I went to a UNESCO art exposition which practically crucified me . . . never before have I seen so many bad paintings all at one time. If I had lived in a bordell (sic) for two weeks I would have been in better condition. I saw a thing of Gillies there, mediocrity personified, in the British section, thank God Scotland is not mentioned.''

He also displays less cerebral concerns about finding accommodation and, more particularly, affordable rations in the black market economy of the late forties. And he writes to Fergusson from Paris with a new wheeze involving Fergusson sending a note on suitably official-looking paper confirming that Bain is the assistant art editor of Scottish Art and Letters. ''Therefore I can successfully acquire a sort of journalistic status and be eligible for rations from the British embassy as well as French.'' Margaret Morris's contacts also took Bain south to St Paul de Vence, where he was invigorated as a previous generation of Scots had been, by the radiance of the light and colour and the agreeable climate. Even his later works in Scotland incorporate that vibrancy. His letters from the south to Fergusson are also agreeably gossipy.

''29th July 1947. I met three guests who are friendly toward me and they asked me to come and have a drink. I did and there was Picasso one table from us looking very well and strong. He had on a white woollen jersey and shorts. I am told that he is very helpful to artists. (Matisse is not.)'' Despite the lean and sometimes lonely times, Bain enjoyed patronage not just from Fergusson but from the Singleton family and Dr Tom Honeyman who bought work for the Glasgow art gallery. His fan club continues today with the N S Macfarlane Charitable Trust giving a grant for a catalogue which is, with the inclusion of so many colour plates and the Fergusson letters, something of a collectors' item.

n Donald Bain, A Scottish Colourist, is at the William Hardie Gallery in Glasgow until May 6.