Which of the following colours connects hits by the artists Joni Mitchell, Tony Orlando and Dawn, and Donovan?

The answer is: Yellow

"Mellow Yellow" is a song written and recorded by Scottish singer-songwriter Donovan. It reached No. 2 on the US Billboard Hot 100 in 1966 and No. 8 in the UK in early 1967.

"Big Yellow Taxi" is a song written, composed, and originally recorded by Canadian singer-songwriter Joni Mitchell in 1970, and originally released on her album Ladies of the Canyon. It was a hit in her native Canada (No. 14) as well as Australia (No. 6) and the UK (No. 11). It only reached No. 67 in the US in 1970, but was later a bigger hit there for her in a live version released in 1974, which peaked at No. 24.

"Tie a Yellow Ribbon Round the Ole Oak Tree" is a song by Dawn featuring Tony Orlando. It was written by Irwin Levine and L. Russell Brown and produced by Hank Medress and Dave Appell, with Motown/Stax backing vocalist Telma Hopkins, Joyce Vincent Wilson and her sister Pamela Vincent on backing vocals. It was a worldwide hit for the group in 1973.