On 10/01/2019 20:17, tim... wrote:
Went into a store (I won't name it because it mighty not be the store's
fault, could be a generic problem with this product) with a sample of
the emulsion paint that I wanted, painted on to a piece of plasterboard
(of the same type that I expect to be painting onto).

now that I have opened the tin and painted the the wallsÂ* (well a bit
of) the colour has dried out to be closer to that of the base colour of
the plaster board than the colour that I pained on.

Try a second coat on the same area before giving up. The first coat is
likely to still show some of whatever is underneath it through.

I guess that the emulsion is translucent to the laser device that they
use to determined the colour mix and I got a result tainted by the
colour of the backing?

Emulsion tends to need (at least) two coats to entirely obscure the
material that it has been painted onto.

going back tomorrow with the same painted onto some gloss white paper.

bugger is I bought 10L of it

Oops. Colour matching is something of a challenge. I prefer to take away
shade cards and interpolate the formula. I have used this to good effect
matching existing faded known composition paints in our VH. It is tricky
because the paint systems may have the same name but have changed over
the years as new resins and low VOC formulations have become the norm.

--
Regards,
Martin Brown