In November 1959 the Yamaha factory developed a new racing YDS1 based engine under the code name YX24 250S, designed by a talented Engineer named Hasegawa, who later went on to become the Chief of Yamaha. It continued to adopt some of the YD's technologies such as polished and knife edged conrods, milled billet cranks, twin mikuni carburettors, magneto ignition and piston porting, but bore and stroke was now set at 50 x 56mm the same as the YD-B. Some special works engines even featured lightweight magnesium crankcases instead of aluminium alloy.  

During 1959, things became very busy in the Yamaha racing department! As well as the "limited-production" YDS1 Asama Clubman, the Factory were busy developing new 100% dedicated factory road racers.
 
In late 1959 - 1960 Engineers Hata and Naito began developing 125 and 250cc machines based on the disk valve racer which Showa had produced for the 1959 Asama races. A 125cc prototype machine with single disk induction was developed first coded YX18, a twin induction 125 machine coded YX24 soon followed. 

The YX18 featured a frame developed from the old YD-A chassis. New upgraded brakes, sleeker and lighter alloy fuel tanks and seats were also developed for the YX18 along with other more contemporary designed parts.

A "doubled up" 125 became a new 250cc rotary engine later in 1960 which was coded YX48 and was fitted in a similar frame, known as the YX47.

As well as housing the new disk valve engines, 
Yamaha had been developing a racing version of their new 250cc commercial piston ported twin called the YX24 250S and quite logically fitted this engine into a similar YX47 chassis. This YX24 250S/YX47 machine became known as the RR or RR 250.  A prototype works version was also built with magnesium engine cases (which, if it had been put into full production in keeping with Yamaha's later naming convention, would have been known as the RD24). These piston ported works machines were raced in several Asian events such as the Singapore GP by Wong Fook Kwong and Albert Lim, as well as making an appearance at the 1961 Daytona GP in February 1961 ridden by Fumio Ito and Yoshi Sunako. The bike was finished in Yamaha's chosen colour scheme of the period, black, but with a "red" fairing (see notes below) and seat was chosen for Daytona and as seen in some advertising. Several US riders such as Dave Buising and Tony Murphy were also lucky enough to trial the bike and give their feedback to the company. Later in 1961 10 customer "race ready" versions of the RR machines were shipped into the US, later to become known as "Yellow Tankers", although this batch of machines featured an improved version of the YX47 frame and rear brake hub. 

Although surprisingly it has recently been discovered by the author that these machines were not originally intended to be supplied in Yellow by Yamaha. 
Original paint found under some Yellow on an original seat unit, clearly shows the Japanese factory originally painted the seat a similar red-brown colour as previous YDs1R fuel tank.   
The rumours were that West Coast Yamaha distributer Frank Cooper chose yellow for the bikes as that was his favourite colour, but perhaps now we can guess why ‘someone’ made a decision to have them repainted… Maybe "they" (Cooper/Caswell) thought brown would not be a great selling factor and “Brown Tanker” just didn’t have the same ring to it !
There maybe another answer though, the Daytona bikes were painted with a "red" seat and fairing, although I have never seen a colour photo of these machines, could the red-brown of this seat be the red of a Daytona machine ?  A colourized image below shows a more bluey-red rather than brown, but this could of course be misinterpreted. I'd very much like to find out for sure.