Thomas Girtin was a friend and rival of Turner's; they were both born in the City of London, two months apart in 1775, and met as teenagers when they were both employed to colour prints with watercolours. Girtin is considered to have been one of the first masters in the use of watercolour. He died in 1802, aged 27, either of tuberculosis or during an asthma attack; Turner survived him by 49 years, and (as history records) became one of Britain's most celebrated artists.
This remark is not recorded in any of my dictionaries of quotations (Oxford, Penguin), but Wikiquote does have a verified entry: "I would at any time have given one of my little fingers to have made [one of Girtin's yellow drawings]."
According to a page on the website of Lancaster University, the "I should have starved" remark is recorded in an 1877 biography of Turner (who died in 1851). The biography in question is The Life of J. M. W. Turner, by Walter Thornbury.
The website of the auctioneer Christie's has the remark as "Had poor Tom lived, I would have starved."
(The Lancaster page is part of an "electronic edition" of Modern Painters, a five–volume work by the eminent Victorian art critic John Ruskin, which was completed in 1860.)
© Haydn Thompson 2020