Phosphine oxide photoinitiator systems and curable compositions with low color

A process for curing ethylenically unsaturated polymerizable compounds, wherein minimal yellowing occurs upon curing, which comprises

• adding to said compounds
• (a) at least one acylphosphine oxide photoinitiator of formula (I)
• in which R1 and R2 independently of one another are C1-C12 alkyl, benzyl, phenyl which is unsubstituted or substituted from one to four times by halogen, C1-C8 alkyl and/or C1-C8 alkoxy, or are cyclohexyl or a group COR3;
• or
• R1 is —OR4, or a group
• R3 is phenyl which is unsubstituted or substituted from one to four times by C1-C8 alkyl, C1-C8 alkoxy, C1-C8 alkylthio and/or halogen, or is a group
• R4 is C1-C8 alkyl, phenyl or benzyl;
• Y is phenylene, C1-C12 alkylene or cyclohexylene;
• and
• X is C1-C18 alkylene or a group
• (b) one or more lightfast red, blue or violet organic pigments
• wherein the weight ratio of component (b) to component (a) is from about 10 parts per million to about 10,000 parts per million, and
• wherein a dispersion of the pigment or pigments of component (b) in a liquid has a positive a value and a negative b value on the L, a, b tri-stimulus color scale,
• and irradiating the mixture so obtained with ultraviolet radiation or daylight or with light sources equivalent to daylight.