providing you could find a bath big enough or would the colour be uneven? Is it just limited to certain aluminium alloys? Say if you bought a second hand frame got the paint off could you do that.

Generally, yes.
Tho' a guy at work had something he'd machined, then assembled using some sort of low temp ally which did something funny (went black?) in the bath used to strip the oxide – probably something to do with that particular alloy. Quite unlike what's used in welding, tho'

Yeah, depends on the quality of the alloy. Be careful though, i had some suspension links annodised and the process ate the threaded parts meaning i couldn't put them back together 🙁

For thin sheet aluminium it should be grade JS57 to get the best finish and for extrusions/tubes and heavy lumps of aluminium it should be grade 6063T6 although your unlikely to find these alloys in aluminium used for bike tubing.
Not much help, but the answer really is in the quality of the pre treatment and the anodising plant.

We've just had some bits anodised where i work and two of the bits came back about 1.5mm smaller across once face (part approx 30*20*10mm) and deeply pitted – a threaded hole on the face was very damaged & crumbly.
Anodising company described it as 'burnt' and said that it was the grade of aluminium that the machining company had used.

Just to throw some light on Stumpy's post T6 is a hard alloy which may explain the pitting and damage.

Just to throw some light on Stumpy's post T6 is a hard alloy which may explain the pitting and damage.Caused to Stumpy's allow ie grade of material not hard enough