Hey no problem or offense taken Rob_K..........none at all.
It's interesting when you mention different types of white sheds/walls and their possible influence on the spectral lines........never thought of that
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White paint has : Titanium dioxide, calcium, silica, zinc oxides, talc,binders etc.(all depending on what type of paint is used and the condition or oxidisation level).
So, spectroscopes are designed to reveal these elements and any others that make the so called ''white light''.
A reflective needle will be far truer towards ''solar light'' than a white painted wall no doubt
I will repeat this little exercise with a shiny needle and try to photograph the results using a CD based spectroscope.