What does that even mean "be a good liar" ? It means that in order to convincingly tell something that is not true, you have to add some really believable true elements to your statement. Like :" Guess who sat beside me on the Earls" patio on Main street, it was (insert famous person name)" . Because you included a bit of verifiable truth (the Earl's on Main), your false statement (in reality no famous person came to town;( ) appears more believable. So, it's the same for painting. You want to use wild crazy colours in your landscape or cityscape or portrait, it will be more believable if you maintain some truth, in this case, truth as perspective and proportions. Viewers know when roads are drawn all wrong, like not going smaller in the distance, or trees don't make sense with the main trunk smaller than the branches! so pay attention to those details and then enjoy going wild with the colours or other elements of design.