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I did two different dilutions of $\ce{FeCl3}$ (within a $\pu{1h}$ timespan) from $\pu{0.05 M}$, and the one with $\pu{5E-4 M}$ concentration looked a darker orange than the one with $\pu{5E-3 M}$.

The picture shows on the left: $\pu{5E-3 M}$ (I erased the label on the flask); on the right: $\pu{5E-4 M}$

FeCl3 Dilutions

I did a test with litmus paper, which showed that the more concentrated one ($\pu{E-3}$) was more acidic, which makes sense. I also did a rates test ($\ce{FeCl3 + H2O2}$), and the more concentrated one made the reaction happen faster, which again makes sense. Why, then, does the more concentrated $\ce{FeCl3}$ look lighter coloured?

Since I had made the more dilute one ($\pu{E-4}$) before, I thought there might have been more time for the ferric hydroxide to precipitate and form a darker colour. But I had only left it for $\pu{1h}$. I also thought that since the more concentrated one looked greenish, $\ce{FeCl2}$ might have been made, yet this does not seem plausible as $\ce{FeCl3}$ is more stable.

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    $\begingroup$ FeCl3 hydrolyses if pH is too low, so if you non acidified solution dilute it too much, it precipitates. I.e. it's darker because of precipitate of Fe2O3 (aq). $\endgroup$ – Mithoron Nov 9 '18 at 19:43

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