In the present day, Mario is always presented with the following colours:
As you can see, his main outfit is red, his trousers are blue and his hat is red.
However, if we look for example at the cover of Super Mario Bros. 2, this is what we can see:
Here we can see that his outfit is blue and his trousers are red, which is the opposite of what it is now.
If you look at the following question and its answers about the original Donkey Kong game, it seems apparent that this used to be his original colour pattern:
As far as I know, the first swap happened after Super Mario Bros. 3, and up until now it became his official colour pattern:
Did they ever explain why they decided to swap the colours?
The same also happened with Luigi apparently. See the following ending screen for Luigi in Super Mario Bros.: The Lost Levels on the SNES:






