TL;DR
The Singapore flag could have been blue.1 It was one of several proposed designs that had been put forward by founding father Deputy Prime Minister Toh Chin Chye (10 December 1921 - 3 February 2012) and his committee.2 Eventually, another design – red and white with five stars and a crescent – was picked. Toh, who was the People’s Action Party’s (PAP) founding chairman, had been given the task of developing Singapore’s official symbols to reflect the values and identity of the newly-formed self-governing state.
Apart from the flag, Dr Toh also contributed to the design of the state crest3 and worked on the national anthem with composer Zubir Said4
Notes
1 https://www.straitstimes.com/singapore/flying-high
2 https://eresources.nlb.gov.sg/infopedia/articles/SIP_488_2004-12-18.html
3 https://eresources.nlb.gov.sg/infopedia/articles/SIP_647_2005-01-11.html?s=toh%20chin%20chye
4 https://eresources.nlb.gov.sg/infopedia/articles/SIP_815_2004-12-23.html?s=national%20anthemn
5 https://www.pmo.gov.sg/newsroom/prime-minister-lee-hsien-loongs-condolence-letter-demise-dr-toh-chin-chye
6 https://eresources.nlb.gov.sg/infopedia/articles/SIP_647_2005-01-11.html?s=toh%20chin%20chye
7 https://www.asiaone.com/News/Latest%2BNews/Singapore/Story/A1Story20120204-325912.html
8 https://www.asiaone.com/News/Latest%2BNews/Singapore/Story/A1Story20120204-325912.html
9 https://eresources.nlb.gov.sg/infopedia/articles/SIP_647_2005-01-11.html?s=toh%20chin%20chye
10 https://www.pmo.gov.sg/newsroom/prime-minister-lee-hsien-loongs-condolence-letter-demise-dr-toh-chin-chye
11 https://eresources.nlb.gov.sg/infopedia/articles/SIP_2015-02-04_094340.html
12 https://eresources.nlb.gov.sg/infopedia/articles/SIP_662_2005-01-11.html
13 Kevin Tan & Lam Peng Er (2018). Lee’s Lieutenants: Singapore’s Old Guards. Singapore: Straits Times Press, p. 46
13 https://eresources.nlb.gov.sg/infopedia/articles/SIP_2015-02-04_094340.html
14 https://eresources.nlb.gov.sg/infopedia/articles/SIP_2015-02-04_094340.html
15 https://eresources.nlb.gov.sg/infopedia/articles/SIP_647_2005-01-11.html
16 https://eresources.nlb.gov.sg/infopedia/articles/SIP_647_2005-01-11.html?s=toh%20chin%20chye
17 ; https://eresources.nlb.gov.sg/infopedia/articles/SIP_647_2005-01-11.html
18 https://eresources.nlb.gov.sg/infopedia/articles/SIP_647_2005-01-11.html?s=toh%20chin%20chye
19 https://eresources.nlb.gov.sg/infopedia/articles/SIP_54_2005-01-25.html; https://eresources.nlb.gov.sg/infopedia/articles/SIP_488_2004-12-18.html?s=national%20flag; https://www.asiaone.com/singapore/spore-founding-fathers-dr-toh-chin-chye-ensured-spore-flag-was-all-inclusive
20 https://eresources.nlb.gov.sg/infopedia/articles/SIP_815_2004-12-23.html?s=national%20anthemn
21 https://eresources.nlb.gov.sg/infopedia/articles/SIP_647_2005-01-11.html?s=toh%20chin%20chye
22 https://eresources.nlb.gov.sg/infopedia/articles/SIP_647_2005-01-11.html?s=toh%20chin%20chye
23 https://eresources.nlb.gov.sg/infopedia/articles/SIP_50_2005-01-17.html
24 https://eresources.nlb.gov.sg/infopedia/articles/SIP_647_2005-01-11.html?s=toh%20chin%20chye
25 https://eresources.nlb.gov.sg/infopedia/articles/SIP_647_2005-01-11.html?s=toh%20chin%20chye
26 https://eresources.nlb.gov.sg/infopedia/articles/SIP_647_2005-01-11.html?s=toh%20chin%20chye
27 https://www.nas.gov.sg/archivesonline/audiovisual_records/record-details/51f06bc5-1164-11e3-83d5-0050568939ad