Advice needed on repainting over a darker colour

[KarenHL]

The room is dark and gloomy, with sticker marks on the wall (don't know if it's oil or adhesive).

In the past (another house) I paid someone to paper over in lining paper, then did 2 coats of undercoat. On a tighter budget now.

The colours now are similar to:
3 walls in this blue
And one wall in sapphire salute

It was the previous owner's teen boy room - and probably gets the least light of all rooms.

I want to paint in this colour.

I've already bought this lampshade which I love.

If I use an undercoat, does anyone have any recommendations?

Otherwise I'll need to try and raise the money sell something to get it lining papered.

[wonkylegs]

The previous owner of our house liked 'strong' colours, the rooms I decorated myself I undercoated in brilliant white before doing the final coat in the colour I wanted.
Some rooms took a few more coats than others - the salsa red (www.dulux.co.uk/en/products/all?id=49230&color-name=Salsa%20Red&color-rgb=AB1D2C&fl-stickey=1&color-id=1187934) kitchen was particularly painful (5 coats) as was the purple bedroom where the walls, woodwork, fireplace and ceiling were all different shades of purple.

[Tugtupite]

It's worth trying a stainblock primer with a tint exact opposite of the colour wheel (from existing shade) to cut out the impact of the original colour when you intent to overpaint to paler colour, like:

1. tinted stainblock
   
2. ordinary primer / undercoat
   
3. 2 topcoats new shade