Renewable lignocellulosic fibers are used more and more in the preparation of eco-friendly materials. Among the existing natural resources, the use of a sterile and non-invasive crop namely miscanthus x giganteus (MxG) [1] is of great interest and could be used in PP polymeric matrix as a green material design perspective. Hence, the present work aims to investigate the consequence of the chemical composition on the color modification of MxG in comparison to flax fibers and their incorporation in a commercial stabilized PP-based blue matrix using an accelerated artificial UV weathering up to 20 weeks. Results using a spectrophotometer show a lightening effect for native and extractive-free flax and darkening for native and extractive-free miscanthus. This behavior could be related to fibers cracking caused by the elimination of degradation products as confirmed by SEM observations. The impact on color perception is also related to the lignin content which is much important for miscanthus (23 wt%) compared to flax (9 wt%). Indeed, lignin is subjected to photocleavage and its transformation to colored chromophore groups (e.g. quinoids) or uncolored groups (e.g. quinoids transformation into carboxylic acid) [2] as confirmed by the ATR-FTIR analysis. Delignified fibers yellowing is related to residual lignin and holocelluloses photochemical degradation. Native and extractive-free PP composites show color enlightenment and disappearance of superficial black spots. These dark spots present before weathering are less important for miscanthus and seem to be caused by a higher thermal degradation of flax during the transformation process. This is explained by the better thermal stability of the miscanthus fibers as confirmed by TGA analysis which led to better color stability than flax. Delignified miscanthus and flax fibers composites show better color stability compared to the pure PP-based stabilized matrix.