We present the results of a study investigating the rest-frame ultra-violet ( UV ) spectral slopes of redshift z \approx 5 Lyman-break galaxies ( LBGs ) .
By combining deep Hubble Space Telescope imaging of the CANDELS and HUDF fields with ground-based imaging from the UKIDSS Ultra Deep Survey ( UDS ) , we have produced a large sample of z \approx 5 LBGs spanning an unprecedented factor of > 100 in UV luminosity .
Based on this sample we find a clear colour-magnitude relation ( CMR ) at z \approx 5 , such that the rest-frame UV slopes ( \beta ) of brighter galaxies are notably redder than their fainter counterparts .
We determine that the z \approx 5 CMR is well described by a linear relationship of the form : { d } \beta = ( -0.12 \pm 0.02 ) { d } M _ { UV } , with no clear evidence for a change in CMR slope at faint magnitudes ( i.e . M _ { UV } \geq - 18.9 ) .
Using the results of detailed simulations we are able , for the first time , to infer the intrinsic ( i.e .
free from noise ) variation of galaxy colours around the CMR at z \approx 5 .
We find significant ( 12 \sigma ) evidence for intrinsic colour variation in the sample as a whole .
Our results also demonstrate that the width of the intrinsic UV slope distribution of z \approx 5 galaxies increases from \Delta \beta \simeq 0.1 at M _ { UV } = -18 to \Delta \beta \simeq 0.4 at M _ { UV } = -21 .
We suggest that the increasing width of the intrinsic galaxy colour distribution and the CMR itself are both plausibly explained by a luminosity independent lower limit of \beta \approx - 2.1 , combined with an increase in the fraction of red galaxies in brighter UV-luminosity bins .