I've noticed this on the EGLM variants of the Katiba, MX 3GL Rifle, MK20 and also on the Skorpion Evo SMG, and there are probably some more weapons in which this happens.
The nonremoveable ironsights which appear while scoped, have the color of the sky
(it appears both when using 1x zoom and 4x zoom) {F20137} {F20138} {F20139} {F20140} {F20141} {F20142} {F20143} {F20144} {F20145} {F20146} {F20147} {F20148} {F20149} {F20150}