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Using RXTE/ASM archival data, we investigate the behaviour of the spectralhardness ratio as a function of X-ray luminosity in a sample of six transientX-ray pulsars (EXO 2030+375, GX 304-1, 4U 0115+63, V 0332+63, A 0535+26 and MXB0656-072). In all sources we find that the spectral hardness ratio defined as$F_{5-12\mathrm{keV}}/ F_{1.33-3\mathrm{keV}}$ increases with the ASM flux(1.33--12 keV) at low luminosities and then saturates or even slightlydecreases above some critical X-ray luminosity falling into the range$\sim(3-7)\times10^{37}$~erg~s$^{-1}$. Two-dimensional structure of accretioncolumns in the radiation-diffusion limit is calculated for two possiblegeometries (filled and hollow cylinder) for mass accretion rates $\dot M$ranging from $10^{17}$ to 1.2$\times 10^{18}$~g s$^{-1}$. The observed spectralbehaviour in the transient X-ray pulsars with increasing $\dot M$ can bereproduced by a Compton saturated sidewall emission from optically thickmagnetized accretion columns with taking into account the emission reflectedfrom the neutron star atmosphere. At $\dot M$ above some critical value $\dotM_{cr}\sim (6-8)\times 10^{17}$~g~s$^{-1}$, the hight of the column becomessuch that the contribution of the reflected component to the total emissionstarts decreasing, which leads to the saturation and even slight decrease ofthe spectral hardness. Hollow-cylinder columns have a smaller height than thefilled-cylinder ones, and the contribution of the reflected component in thetotal emission does not virtually change with $\dot M$ (and hence the hardnessof the continuum monotonically increases) up to higher mass accretion ratesthan $\dot M_{cr}$ for the filled columns.

On the dependence of the X-ray continuum variations with luminosity in accreting X-ray pulsars