How create an 8-bit screen that ObtainBestPen() will work on? (SOLVED)

The AutoDocs for OpenScreen() are not very helpful, and I am having trouble working out what magic tag(s) I need to get ObtainBestPen() working on my custom 8-bit screen. (BTW, I do not want to use SA_LikeWorkbench, even though that appears to work.)

"SA_Pens,-1" seems to get it *slightly* working, such that a tiny handful of colours get allocated (perhaps 7), but it looks pretty horrid.

I'd appreciate advice, as it appears that colour palettes are one of the Amiga's most messy subjects (seemingly having slowly evolved over time).

AFAIK any 8-bit screen should cope with ObtainBestPen(), even 32-bit screens should work. What exactly isn't working?

@Chris Quote:
That's what I had imagined, but that doesn't seem to be the case . It *does* work on 24-bit screens though, which is what had lulled me into a false sense of security. It also *does* work when using SA_LikeWorkbench, but I can't use that for what I want.

Quote:
Without any special tags, ObtainBestPen() does not seem able to allocate any new pens, so it just returns pen 18 for any requested colour (which gives me a completely black screen when I am drawing).

When using "SA_Pens,-1", ObtainBestPen() appears able to allocate at least 6 pens, so that it returns pens 18, 11, 10, 12, 4, 8 & 7. This gives me very poor quality drawing, but at least I can see something!

OK, it turns out that I need to use "SA_SharePens,TRUE" for ObtainBestPen() to work.

I don't really understand why, but it seems to be for maintaining backwards compatibility with OS3.0 or earlier. If you don't use that tag, then Intuition marks all non-system-allocated pens (everything in my case) as being "exclusive", which I *think* means they are marked as being already 'allocated' (for use exclusively by the screen's owner, probably poking colours directly into the palette).