That is a rather sweeping statement, which seems to be a result of distant memories playing tricks as one gets older.
The statement is nothing to do with "distant memories playing tricks" - it is based upon observations at places such as Witney, Oxford and Lancaster, and on the evidence of 35 mm colour slides taken during the period 1969-1973. As far as the Witney branch was concerned, blue class 22s were predominant by 1968/69, and every railtour that visited the branch in its final months of operation was in blue or blue & grey livery. The last green unit to visit the branch was a class 117 set on 14 September 1968. Interestingly, some of the Warships used on the OW&WR route were still in maroon livery circa-1968, and they were hauling blue & grey coaches.
Unfortunately, using a few unrepresentative branch lines with a regular blue diesel as local power for trains does not tally with the wider picture.
As for colour slides, one would have to know if the photographer was deliberately concentrating on newly painted blue locomotives, rather than the green of the "old regime", then seen as outdated and associated with the steam era. You can watch old steam videos of NW England and believe that diesels hardly existed in 1967/8, mainly because the cameramen (with one or two exceptions) did their utmost to avoid filming the "boxes on wheels".
Lancaster would have seen more than its fair share of class 50s in those days, locos that were delivered in blue when new, so that is not representative of the pattern elsewhere, either.
Anyway, there are plenty of Railway Observer magazines which show the true picture, and prove the statement "in 1970 there were very few green engines or dmus left" to be incorrect.