Here’s a great little design truism, gleaned from a paper by cognitive scientist Kyle Cave:

“In our visual system, attention is distributed in the sequence:

1. location
2. color
3. shape”

…from a post to the CHI-web mailing list by Rummel Bernard. Here is the paper: Cave, K.R. (1999) The FeatureGate model of visual selection. Psychological Research 62, 182-194.