Is it correct to say "for optimal user experience" when referring to how web interface has to be designed? I just need to explain why I chose specific colour scheme.
In the case of this colour scheme, mention that it isn't too hard to look at. A bright yellow background would be hard to stare at. Note the colour scheme of this very webpage. The formatting is elegant and boxes jump out at you, but only to welcome you. Not to blind you. If your background is green, you can also say that the green is synonymous to money to give the user a rich feeling and experience. Maybe
Edit: Wordings such as pleasing to the eye and not jarring can be used to justify your choice of colour scheme. Courtesy of J.R.
@AndrewLeach and @J.R. gave a couple of good pointers, but let me steer the ship a bit further and expand or possibly go in a slightly different direction with what they mentioned.
Optimal means the best choice using certain criteria under certain conditions. Some criteria may or may not be directly quantitative, but can be qualitative/subjective preferably if the criteria originates from a trusted and accepted source for guidance. Hence, if you were to suggest that...
... we could not only take your word, but further, we could review, verify (make sure what you claimed was actually stated), and validate (affirm or agree with) the conclusion of the guidance.
To clarify, a "trusted and accepted source" leaves a lot of leeway in practice as to what qualifies as acceptable and what does not. What constitutes a "trusted and accepted source"? It is up to the readership to determine if the standard is acceptable. For all intents and purposes, you could claim...
This takes the form of expert-opinion or testimony.
In your particular case, there was certainly a reason why you feel the choices made were optimal, so you simply need to substantiate them with the method or criteria used to make that determination.