  Continuum spectrum from black hole accretion disc holds enormous information
regarding the strong gravity regime around the black hole and hence about the
nature of gravitational interaction in extreme situations. Since in such strong
gravity regime the dynamics of gravity should be modified from the \EH one, its
effect should be imprinted on the continuum spectrum originating from the black
hole accretion. To explore the effects of these alternative theories on the
black hole continuum spectrum in an explicit manner, we have discussed three
alternative gravitational models having their origin in three distinct
paradigms --- (a) higher dimensions, (b) higher curvature gravity and (c)
Horndeski theories. All of them can have signatures sculptured on the black
hole continuum spectrum, distinct from the standard general relativistic
scenario. Interestingly all these models exhibit black hole solutions with
tidal charge parameter which in these alternative gravity scenarios can become
negative, in sharp contrast with the Reissner-Nordstr\"om black hole. Using the
observational data of optical luminosity for eighty Palomer Green quasars we
have illustrated that the difference between the theoretical estimates and the
observational results gets minimized for negative values of the tidal charge
parameter. As a quantitative estimate of this result we concentrate on several
error estimators, including reduced $\chi^{2}$, Nash-Sutcliffe efficiency,
index of agreement etc. Remarkably, all of them indicates a negative value of
the tidal charge parameter, signaling the possibility of higher dimensions as
well as scalar charge at play in those high gravity regimes.
