El Cid wrote:grenadiers have grey coats with blue trousers
Hoffmann pictures blue coats for the Grenadiers all the way till 1727
Yes, but Hoffmann pictures only a Grenadier in the year 1694, then again 1717 (at least in my version) - the grey coats were given out from 1698 till 1702. Then they got again a blue coat - in the same year the colours of the cuffs of all the regiments have changed again. If I find time I will write these changes down.
the grey coats seem to be speculative at best or confined to one or two regiments
Well, partly. The Info is given for the Leibregiment but in conjunction with the re-provision of new uniforms for the five regiments serving in the Netherlands - so less speculative than not paying attention to this change. Also the info is written in files in the War Archive (Protokolle, Kriegsrat), so - with a high probability - it is true.
Then again - we do not know if all the uniforms have been given out, all the changes had been performed untill the next order changed the appereance of the uniform etc.
So for painting regiments in miniature I would need to paint the grenadier companies in grey coats, is that for ALL bavarian regiments?
Well, if I were in your place and would like to display the army during the timeframe of the years (before the War of Spanish Succession), then I would paint one or two bases with grenadiers grey, the rest blue. - I would see the army more as a collage.
Btw: If forgot to write down in my last post that Harthausen had red stockings in 1695, Leibregiment and Rivera white-grey.