What is the relative importance of shape and color in object recognition? Do people recognize objects faster when both shape and color are present? Can color introduce more cognitive load to identify the object?
Let's say there is a row of icons. Users need to quickly (no more than 3 seconds) decide which icon to click on. Which icon configuration would lead to quicker decision making? 


shape only (no color) 
shape and color


Do people react to color or shape first?
People are generally faster to find a dissimilar stimulus when it differs along two dimensions (color+shape) from the rest, but only if they are looking for those dimensions (e.g. find all objects that are red diamonds is faster than find all objects that are red). However, if they are looking for dimensions which are not present, this effect reverses (e.g. find all objects that are not blue is faster than find all objects that are not blue circles). 

The reasons behind this are still a matter of debate, but one model (Buckingham et al, 2012) suggests that the best explanation is that both dimensions are processed simultaneously. See here for more information on why this model fits the data: Two perceptual anomalies explained by a statistically optimal model.

Buckingham, S., de Gardelle, V., Avery, S., &amp; Summerfield, C. (2012). Two perceptual anomalies explained by a statistically optimal model. Journal of Vision, 12(9), 1053-1053.