  Quantitative data is frequently represented using color, yet designing
effective color mappings is a challenging task, requiring one to balance
perceptual standards with personal color preference. Current design tools
either overwhelm novices with complexity or offer limited customization
options. We present ColorMaker, a mixed-initiative approach for creating
colormaps. ColorMaker combines fluid user interaction with real-time
optimization to generate smooth, continuous color ramps. Users specify their
loose color preferences while leaving the algorithm to generate precise color
sequences, meeting both designer needs and established guidelines. ColorMaker
can create new colormaps, including designs accessible for people with
color-vision deficiencies, starting from scratch or with only partial input,
thus supporting ideation and iterative refinement. We show that our approach
can generate designs with similar or superior perceptual characteristics to
standard colormaps. A user study demonstrates how designers of varying skill
levels can use this tool to create custom, high-quality colormaps. ColorMaker
is available at https://colormaker.org
