An optical waveguide (1300a, 1300b) comprising: a light-transmissive substrate (1350a, 1350b) configured for use in a near-eye display, NED, device, the substrate (1350a, 1350b) including a plurality of internally reflective surfaces, the substrate made of a first material having a first refractive index; and a diffractive optical element, DOE, (1360a, 1360b) formed on a first surface of the plurality of surfaces of the substrate (1350a, 1350b), the DOE (1360a, 1360b) configured to input light rays to the substrate (1350a, 1350b) or output light rays from the substrate, the DOE including: a diffraction grating (1370a, 1370b) made of a second material having a second refractive index; and a coating (1380a, 1380b) over the diffraction grating (1370a, 1370b) made of a third material having a third refractive index; wherein the second refractive index is not equal to the third refractive index; and wherein the optical waveguide includes at least three further DOEs (762b, 764b, 766b), each including the diffraction grating (1370a, 1370b) and the coating (1380a, 1380b), the at least four DOEs being formed on two opposite surfaces (752b, 754b) of the substrate that are parallel to the direction of propagation of light rays within the substrate, wherein the two opposite surfaces (752b, 754b) comprise the first surface, wherein the DOEs on one surface of the waveguide couple a first color and the DOEs on the opposite surface of the substrate couple a second color by the DOEs working only on one polarization or using switchable DOEs to enable selection of the coupled color for each DOE. a light-transmissive substrate (1350a, 1350b) configured for use in a near-eye display, NED, device, the substrate (1350a, 1350b) including a plurality of internally reflective surfaces, the substrate made of a first material having a first refractive index; and a diffractive optical element, DOE, (1360a, 1360b) formed on a first surface of the plurality of surfaces of the substrate (1350a, 1350b), the DOE (1360a, 1360b) configured to input light rays to the substrate (1350a, 1350b) or output light rays from the substrate, the DOE including: a diffraction grating (1370a, 1370b) made of a second material having a second refractive index; and a coating (1380a, 1380b) over the diffraction grating (1370a, 1370b) made of a third material having a third refractive index; wherein the second refractive index is not equal to the third refractive index; and wherein the optical waveguide includes at least three further DOEs (762b, 764b, 766b), each including the diffraction grating (1370a, 1370b) and the coating (1380a, 1380b), the at least four DOEs being formed on two opposite surfaces (752b, 754b) of the substrate that are parallel to the direction of propagation of light rays within the substrate, wherein the two opposite surfaces (752b, 754b) comprise the first surface, wherein the DOEs on one surface of the waveguide couple a first color and the DOEs on the opposite surface of the substrate couple a second color by the DOEs working only on one polarization or using switchable DOEs to enable selection of the coupled color for each DOE. a diffraction grating (1370a, 1370b) made of a second material having a second refractive index; and a coating (1380a, 1380b) over the diffraction grating (1370a, 1370b) made of a third material having a third refractive index; wherein the second refractive index is not equal to the third refractive index; and wherein the optical waveguide includes at least three further DOEs (762b, 764b, 766b), each including the diffraction grating (1370a, 1370b) and the coating (1380a, 1380b), the at least four DOEs being formed on two opposite surfaces (752b, 754b) of the substrate that are parallel to the direction of propagation of light rays within the substrate, wherein the two opposite surfaces (752b, 754b) comprise the first surface, wherein the DOEs on one surface of the waveguide couple a first color and the DOEs on the opposite surface of the substrate couple a second color by the DOEs working only on one polarization or using switchable DOEs to enable selection of the coupled color for each DOE.