They're not all white, and were painted that way especially for the JCC International Trophy. Handicapping was by way of progressively more difficult parallel chicanes set up at The Fork: "tiddlers" went straight through without deviating, while the largest cars had to negotiate a very tight corner. The colours were applied to the tails so that the officials could check nobody was cheating - depending on which size of engine was fitted to (for example) an ERA, Alta or Maserati 4CM it could be running in any one of three different classes.
This aerial photo shows the 1936 race, with most of the field approaching the handicap lanes, which can be seen top right. If you look carefully, it appears that five(?) cars in the 1750cc class - which was the next-to-largest - have already got away and are just exiting their lane. Two more are negotiating the tightest bend.