Which common color usage mistake is illustrated in this chart?

A scatter chart with colored dots plots actual hours versus estimated hours. The horizontal axis is labeled $?$Estimated Hours? and ranges from $0$ to $120$ with tickmarks in increments of $40.$ The vertical axis is labeled $?$Actual Hours? and ranges from $0$ to $120$ with tickmarks in increments of $40.$ The legend on the right side of the chart shows the color of the dot corresponding to different repair person. There are $20$ repair persons. The dots show an increasing trend between $(5,5)$ and $(90,110).$ All data are approximate.

a$.$ Insufficient contrast

b$.$ Excessive color

c$.$ Unnecessary color

d$.$ Neglecting colorblindness