  The James Webb Space Telescope (JWST) will open a new window of the most
distant universe and unveil the early growth of supermassive black holes (BHs)
in the first galaxies. In preparation for deep JWST imaging surveys, it is
crucial to understand the color selection of high-redshift accreting seed BHs.
We model the spectral energy distribution of super-Eddington accreting BHs with
millions of solar masses in metal-poor galaxies at $z\gtrsim 8$, applying
post-process line transfer calculations to radiation hydrodynamical simulation
results. Ten kilosecond exposures with the NIRCam and MIRI broad-band filters
are sufficient to detect the radiation flux from the seed BHs with bolometric
luminosities of $L_{\rm bol}\simeq 10^{45}~{\rm erg~s}^{-1}$. While the
continuum colors are similar to those of typical low-$z$ quasars, strong
H$\alpha$ line emission with a rest-frame equivalent width ${\rm EW}_{\rm
rest}\simeq 1300~\r{A}$ is so prominent that the line flux affects the
broad-band colors significantly. The unique colors, for instance F356W$-$F560W
$\gtrsim 1$ at $7<z<8$ and F444W$-$F770W $\gtrsim 1$ at $9<z<12$, provide
robust criteria for photometric selection of the rapidly growing seed BHs.
Moreover, NIRSpec observations of low-ionization emission lines can test
whether the BH is fed via a dense accretion disk at super-Eddington rates.
