The parametric factors k_L, k_C and k_H that scale the CIELAB components ΔL*, ΔC* and ΔH* in the CIE94 colour difference formula are unity under reference conditions. When the conditions are changed, the scaling factors may be adapted to account for the influence of specific experimental conditions on perceived colour differences. We determined thresholds for the visibility of static background noise and for the visibility of a test symbol. The noise was present in only one of the L*, C* or H* dimensions, and the test symbol was an increment to the background, also in one of the dimensions L*, C* or H*. In order to maintain a perceptual uniform difference metric between test symbol and noisy background we arrived at k_L = 0.15, k_C = 0.52, and k_H = 2.21, such that a just noticeable difference corresponds to ΔE*₉₄=1. When the dimension (L*, C* or H*) of the incremental test symbol is the same as that of the noise in the background, the threshold for the test symbol increases linearly with the noise. When the dimensions are different, the thresholds for the test symbol remain constant (background noise in L*) or slowly increase (background noise in C* or H*).