A nice coffee (with cream!) brown without any deposits is what you
want....not too light not too dark.

The lighter/whiter the plug is, the leaner/hotter the engine is
running.  If it's white, the plug is too hot, the fuel mix too lean
and/or possibly timing is off.  If it's dark and especially if at all
wet with redidue, the plug is too cold (use what the manufacturer
recommends, of course) the fuel mixture is too rich and/or the timing is
off.  These are the basic rules of thumb.  there are an infinite number
of variations of symptoms and causes within the general rules, though.
Looking at the spark plug is like taking the temp of a child who does
not feel well.  It's a good starting point...it allows you to see what
is happening in the combustion chamber over some time.  It will give you
some indication of whether there is a problem and what it might be, but
it will not answer all questions.
steve

Johan Nyberg wrote:

> Hello all
>
> changed the plugs yesterday. The old ones (NGK BP6ET) are a nice
> brick-brown colour. What can this tell me about the state of the
> engine? My
> repair and maintainance manual only gives different hues of gray, from
>
> sooty black to white, but no browns.
>
>   ************************************************************
>     Johan Nyberg, Sweden N 59o E 17o 25"
>     '86 Caravelle C
>   ************************************************************