Removed old wallpaper - why's it blue underneath?

[BatsPigeonsRatsSquirrels]

At first I thought the blue I was finding under two layers of painted-over wallpaper was old paint, but it's got patches of different colour and I think it's something else - googling suggests people might have put glue or something else under when planning to wallpaper?
I want to remover the paper and then paint the wall. It was a particularly unpleasant paper, imo! Do I need to try to remove the blue stuff (it comes off a little bit with sugar soap but most of it is left behind) or can I paint over it? Should I use a special primer or something? I will attach pictures - including one where a bit of plaster came off with the paper so you can see the wall underneath.
For reference, it's a former local authority house built around the 60s (I think).
Thanks to anyone who replies!

Pics

Could it be distemper?

Problem Solver - Distemper

Form of whitewash normally made using chalk or lime and a size in extreme cases lead was also used . You can identify by wiping your hand over the surface, is a white powder is present it is likely to be distemper.

Yes it’s probably distemper. FWIW I had the same thing stripping a room the other day, house was around 100 years old and the first layer was applied directly to the plaster it seemed. Quite a lovely colour as it goes, it’s the orange in this pic.

Hmm, can't see anything on my hand when I wipe it over the wall (though there has been a lot of water on it to get the paper off!)
If it is distemper, what does that mean for painting?
thanks so much for replying!

Well it could be paint. If you wet the surface in question, it will absorb the water and go into a kind of foamy mush if it’s distemper. If it’s paint, it won’t do this and will be largely unscathed.

I once had the displeasure of scrubbing distemper off a ceiling before plastering. Incredibly messy, but you can shift it with sugar soap and a lot of sponging. Paint you can’t really do this the same, it won’t scrub off.

Oh I'm glad it's not the ceiling! That must have been hard. There's paper on mine but it is just staying there.
I'm going to be cheeky and put out a call for the omniscient @pigletjohn as I'm not convinced about how to paint over it.
Let sleeping walls lie, is my new motto.

In my house its green underneath!

You can scrub off distemper with very hot water but not with cold.

It has a distinctive unpleasant smell when scrubbed off due to being made of boiled-up hooves.

You can't successfully paint over it.

It is generally in pastel colours, in kitchens, bathrooms and WCs due to being fairly resistant to water.

Thanks so much for replying.
Ive tried scrubbing with very hot water just now, there was a little bit of blue on the brush but it hasn't come off the walls.
I don't think it's powdery and it doesn't smell bad (to me, anyway!)
I don't think I can definitively say it is distemper.

Well you could always try and paint a small patch as a test. If it’s not flaked away after a day or two you’ll probably be ok, as PigletJohn says rightly, paint will not stick to distemper.

When I have had decorators in to paint my walls in a plain colour they always paper with lining paper first and paint that. They usually put the paper on horizontally, too. Not an easy job but this is the only way you will get a good finish.

Good ideas, thank you! Wish I could bring you round to have a look in person
I was so proud of myself for a while for getting all the layers of paper off

Agree with PP. It's much better to paint over lining paper than bare plaster in older properties.

Why is that @JamMakingWannaBe ?

Or replaster perhaps?

We usually get early reskin of plaster if we want to paint a room. Much cleaner finish than painting over paper

I don't disagree with anyone re lining paper or replastering, but honestly I just want to put a bit of colour in the room and get rid of very old fashioned embossed paper.
I don't have the budget for replastering, unless the wall is hanging off obviously!
I've to spend a couple of hundred today to get a squirrel out of my attic.
The square I painted last night is so far looking just fine so I might be good with just a decent primer on - but I will leave it a full 24 hours to be sure.

If it won't scrub off with very hot water, it isn't distemper. Wash the wall, and rub it off with old towels to remove dirt and old wallpaper paste, and give it a couple of coats of matt white emulsion. This will highlight to the eye any defects that need filling or smoothing. Once that is satisfactory, you can paint in your chosen colour. Any patches of bare filler or plaster need a couple of mist coats of dilute emulsion first.

I prefer joint cement or plaster, but you can use filler if you have time. Press it very hard with a broad metal filling knife to flatten, and shave off any excess before it hardens. This is better, quicker and easier than sanding. Use the minimum possible thickness, pressed down to nothing.

@PigletJohn that's really helpful, thanks, I will do exactly that.
Is white emulsion preferable to a special primer?

It's a lot cheaper, easy to use and clean up, and except in very unusual circumstances, is all you need.

Matt is easy to paint over, but does not stick so well to gloss or silk unless you take the shine off first, e.g. lightly wiping with fine abrasive.

Great, thanks. Might nip to b&q before it shuts

If your walls have lots of bumps and dips, strong lining paper does a fantastic job of smoothing over it all and gives you a much nicer surface to paint.
Our Victorian walls were a right mess and lining paper made them look much better. I hung it horizontally in one room but gave up and went vertical for the rest!

We had to decorate a room with awful rough plaster and used a lining paper with a very tiny embossed pattern - well more an all-over texture , no pattern as such. Like the paper equivalent of frosted glass I suppose!. It worked really well.

If the walls are uneven, there's a lining paper called Wallrock which is great stuff. It's not cheap, but you can get it in a wider width from Toolstation or Screwfix (cheaper than B&Q) which means you can line the room more quickly than with standard width. Hang it vertically even if you think you might paper the room again in future. You can use caulk over the seams for a seamless finish.

Lots of useful tips on this thread, hopefully might come in handy to someone else. I've finished painting today and am happy with the finish - wall looks smooth and I can't see any of the plaster/odd bits anymore. Did three emulsion coats and two of the colour.
Feel ready to tackle the rest of the house now!
Thanks everyone