A composition comprising a compound of Formula (1) or a salt thereof and compound of Formula (2) or a salt thereof: wherein: A is or comprises an aryl or heteroaryl group; Q1 and Q2 each independently is an optionally substituted phenylene group; R1, R2, Ra and Rb each independently is H, optionally substituted alkyl, optionally substituted aryl or optionally substituted arylalkyl; each W1, W2, W3 and W4 independently is a —CO2H, —SO3H, —PO3H2 group or an alkyl substituted with one or more groups selected from —CO2H, —SO3H, —PO3H2; each S1, S2, S3 and S4 independently is a substituent other than those defined for W1, W2, W3 and W4; a and d each independently is from 1 to 5; b and c each independently is from 0 to 4; (a+b) has a value of from 1 to 5; (c+d) has a value of from 1 to 5; e and h each independently is from 1 to 7; f and g each independently is from 0 to 6; (e+f) has a value of from 1 to 7; and (g+h) has a value of from 1 to 7.