Colour is becoming a baseline requirement in the avionic displays market. Implemented for decades in Head Down Displays (HDD), it is thought to enhance Situational Awareness and minimise errors in decision making. Even though a wide colour gamut can be achieved in eyes-out display devices, its application and its usefulness for symbology effectiveness remains debatable. Reconciling these two issues would significantly improve the standardisation of eyes-out displays, enhancing safety while reducing costs of ownership. However, designing a robust set of colour symbology, for all eyes-out display types and in all conditions of operation - in particular Degraded Visual Environments (DVE) - is less straightforward than in HDD. In fact, the transparency dimension of the display can cause a divergence between the intent of the coloured symbology and its recognition/discrimination by the user. The effectiveness of colour as an attention getter and the associated design constraints for real situations are investigated. This report summarises the main features to take into account when assigning colour to an eyes-out display, including a discussion on the Green/Amber/Red code. The approach suggested aims at developing a model that uses colour symbology effectively in both the aircraft space- and time- frames simultaneously. In colour eyes-out displays, mission performance is clearly dependent on: Display transparency, Information categorization, Colour perception by the pilot. The interaction between these elements is key to designing a coherent set of colour design rules and the paper ends with a set of recommendations for good practice in eyes-out symbology design.