General results of this nature can be found in Cohomology, Section 20.19, Sheaves, Lemma 6.29.1, and Modules, Lemma 17.22.8.
Lemma 30.6.1. Let $f : X \to S$ be a quasi-compact and quasi-separated morphism of schemes. Let $\mathcal{F} = \mathop{\mathrm{colim}}\nolimits \mathcal{F}_ i$ be a filtered colimit of abelian sheaves on $X$. Then for any $p \geq 0$ we have
\[ R^ pf_*\mathcal{F} = \mathop{\mathrm{colim}}\nolimits R^ pf_*\mathcal{F}_ i. \]
Proof. We will apply Cohomology, Lemma 20.19.2. Since the affine opens form a basis for the topology of $S$, it suffices to show that for $U \subset S$ affine open we have $H^ p(f^{-1}U, \mathcal{F}) = \mathop{\mathrm{colim}}\nolimits H^ p(f^{-1}U, \mathcal{F}_ i$. Since $f^{-1}U$ is quasi-compact and quasi-separated we conclude using Cohomology, Lemma 20.19.1. (Because the basis of affine opens in $f^{-1}U$ satisfies the assumptions of that lemma.) $\square$
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Comment #9668 by Jonas Ehrhard on