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

Excavating black hole continuum spectrum: Possible signatures of scalar hairs and of higher dimensions