Rhobs el Arsa
The Rhobs al Arsa has long been viewed as a citron hybrid, though a different cultivars have been suggested for the parent, for example, Moroccan citron as a male parent and sour orange as the female parent, citron or lemon and a mandarin., or citron and the lime. Detailed genomic analysis has shown Rhobs al Arsa to be highly similar to several cultivars of "Citrus limetta", the limetta or Persian sweet lime, and likely shared a common origin with them, arising from a cross between citron ("Citrus medica") and sour orange ("Citrus" × "aurantium"). The common lemon arose from a distinct hybridization event involving the same two species.
Citrus
The colour of citrus fruits only develops in climates with a (diurnal) cool winter. In tropical regions with no winter at all, citrus fruits remain green until maturity, hence the tropical "green oranges". The Persian lime in particular is extremely sensitive to cool conditions, thus it is not usually exposed to cool enough conditions to develop a mature colour. If they are left in a cool place over winter, the fruits will change colour to yellow.