Having searched the 2010 and 2012 Orionbooks.co.uk paperback editions Internet Archive copy of the edition, OP has stated they have an orionbooks.co.uk edition in comments, I am not able to find any occurrences of either 'favorite' or 'color'.
The book was first published in the UK in 1986 by Grafton Books and also in paperback in 2004 by Orion.
if the Op is correct in the orionbooks.co.uk edition having a mixture of 'color' and colour', 'favorite' and 'favourite' it seems likely that this was an incomplete 'translation' to British English from US English, in an earlier edition, which has since been changed. It is possible that there was some rationale for the mixture in that edition, such as direct speech by American characters retaining US spellings, but without information as to the specific edition the OP has, and access to it, it is not possible to say more at this stage.
The fact that the situation pertains in the OP's edition but not in the 2010/2012 editions, suggests that the issue is editorial rather than down to Ludlum's own spelling choices.