TIP: You can search for "Painted" or "Hand-Painted" roses, by selecting "Selection Lists", "Class: Hand Painted Roses.
[From Modern Garden Roses, by Peter Harkness, p. 56:] 'Picasso' was the first in Sam McGredy's series of 'Painted Roses'. (Others: 'Matangi', 'Sue Lawley', 'Eye Paint', 'Little Artist', and 'Maestro'.)
[Ibid, p. 70:] The development of these colours sprang from Sam McGredy's observation of 'Frühlingsmorgen', a shrub rose of five wide pink petals with yellowish centres. This carries genes from ancestral Scotch roses, which are known to have produced 'broken' or 'marbled' varieties in the past.
[From The Complete Book of Roses, Bush Roses and Standards, by Sean McCann, p. 128:] They arrived through a quite staggering train of breeding but [McGredy] feels that the hand painting comes from 'Fruhlingsmorgen', a Kordes shrub from 1942.

[From Peter Schneider on Roses, by Peter Schneider, p. 66:] Sam McGredy had the idea of using hybrids of R. spinosissima (the Scotch briar rose) to bring added winter hardiness to modern garden roses. What he eventually found was that the marbled coloring peculiar to R. spinosissima and its offspring could be captured, intensified and used to create a new family of roses that McGredy dubbed hand-painted... Handpainted roses feature pale streaks and markings emanating from a central eye to agitate a bolder background... White-on-carmine is the most common combination... In all hand-painted roses, the color effect is most striking and dramatic in cool weather.