The 1968 colour information is from the sales material and www.duettoinfo.com While we are at it, the Montecatini was the name of the comapny that manufactured the paint, it is also a lovely town in Tuscany, Italy.
Here you may find the red colour codes used by Alfa Romeo OEM. Rosso is my favourite colour. Roundt-tails also came in white, green, blue, black, silver etc. I have focused on the red colour codes. The code numbers are easy to find, since they are the same used by PPG, DuPont etc. paint shops. Here is a company you may try too: touchuppaintonline
The red colour is the classic Alfa Romeo racing standard. Italian racing cars were always red. Today only (?) Ferrari has the red colour in F1 of the Italian racing teams. It is quite odd to see that Alfa Romeo changed the tone from bright red, close to orange, to the burgundy/blue tone through the years.
You will find the colour code on a small plate under the boot lid to the right. If you respray, any local paint shop will be able to mix the red colour you like if you tell him the OEM #.
The Duetto came in the following colours from factory:
Spider 1.6 duetto 1966-1967
1750 Spider Veloce 1967-1971
Grigio Chiaro Metallizato
*until July 1969
Spider 1.3 Junior 1968-1973
Grigio chiaro met
* until July 1969 ** fStarting November del 1972
2000 Spider Veloce 1970-1977
Grigio chiaro met.
*until September 1976
Spider 1.6/2.0 1980-1983
Grigio chiaro met.
Verde acqua met.
Spider Q.V. 1.6/2.0 1986
Q.V.: rosso e grigio met.
2.0: bianco, rosso, nero e blu medio met.
The 1750 Veloce had these option:
Light Gray Metallic
Pininfarina White or Bianco Pininfarina
Ivory or Avorio
Italia Red or Rosso Italia or ROSSO MONTECATTINI
Pininfarina Blue or Bleu Pininfarina
Sky Blue or Celeste
Metallic Light Grey or Grigio Chiaro Metallizzato
British Racing Green or Verde Inglese
Black or Nero
The complete charts are also sorted by code and by name. The 1968 colour information is here: colour chart, then the Duetto options, the Berlina combinations and the GT Veloce chart.