color

Data type color. Probed on Task.color (stock, editable; written only in the sandbox project); Step.color and Project.color are read here and never written. Those three are the only color fields on the site, and a custom one is impossible: POST /schema/<Type>/fields with "data_type": "color" returns 400 {"data_type": ["data_type is not valid"]} (probe 019). All three report mandatory: false and the same two properties, no vocabulary: default_value (null) and summary_default ('none'). Nothing in the schema tells the three apart.

Read A bare string in attributes, never in relationships. pipeline_step is a sentinel meaning "render this Task with the colour of its Pipeline Step"; resolve it with a dotted path in the same call (probe 003), not a second fetch, and keep a client default for a Task whose step resolves to nothing.

GET /entity/tasks?fields=color,step.Step.color
  {"color": "pipeline_step", "step.Step.color": "253,94,99"}

Write PUT /entity/tasks/<id> with a string; legacy names are case-insensitive and stored expanded.

Each of those 400s appends the same grammar, the authoritative spec for the type:

"Update failed for [Task.color]:  (task.rb) Value is not a valid color token: [#ff8000]Unsupported
 format for Task color field. The Task color can not be nil, the expected format is r,g,b where the
 values of r,g and b are in the range 0-255. The value of the color can also be set using the legacy
 color strings which are; Blue, Orange, Pink, Red, Green, Purple, Grey and Black. ..."

Clear Task.color cannot be cleared; write pipeline_step to un-set it.

After the "" attempt, [["id","is",<id>],["color","is",null]] still matches 0 rows. The refusal comes from the field, not the schema: mandatory: false, and the type layer accepts NilClass. A written null was tried on Task alone, Step and Project rows being site-wide, so nullability on those two is a read-side count and nothing more: 32 Projects hold null, no Step does.

Filter Four relations, byte-identical to list and status_list; contains, not_contains, starts_with and ends_with all 400 with the same body:

[["color", "definitely_not_an_operator", null]] -> 400
 title:  "API summarize() Task.color's 'color' data type doesn't support
          'definitely_not_an_operator' 'relation'"
 source: {"Task.color": " data type doesn't support 'definitely_not_an_operator' 'relation'. Value:
    {"path" => "color", "relation" => "definitely_not_an_operator", "values" => [nil]}
    Valid relations: ["is", "is_not", "in", "not_in"]"}

The value is the stored string, counted over 1900 Task rows:

Step, 35 rows, and Project, 52 rows, reject the keyword form outright and take the triple:

A malformed value 400s on Step and Project and returns 0 rows on Task:

400 "API summarize() invalid format for color value: "#ff8000" Correct format is '127,127,127' with
     values from 0 to 255 inclusive."   same for "255, 128, 0", "red", "255,128"; "300,0,0" gives
     "invalid range in color value"

Traps

• Never treat Task.color as a colour. A swatch renderer parsing it as r,g,b gets pipeline_step on every row this site has and must follow step.Step.color instead.
• Decimal r,g,b, no spaces and no #, matching Status.bg_color (probe 010). Hex is rejected on write and on filter. Build it with "%d,%d,%d" % rgb, split on , to read.
• No Task.color row on this site is empty and a written null is refused, so a fill-rate count over it reads 100% and measures nothing (probe 007).
• A bad filter value is a 400 on Step and Project but a silent 0 rows on Task.
• Writing a legacy name is lossy: "Red" stores 253,1,0 and never reads back as "Red", so filtering on the name it was written with returns 0.

Python equivalent

sg.update("Task", tid, {"color": "255,128,0"})       # decimal triple, no spaces
sg.update("Task", tid, {"color": "pipeline_step"})   # the only way to un-set it; None is a 400
t = sg.find_one("Task", [["id", "is", tid]], ["color", "step.Step.color"])
c = t["step.Step.color"] if t["color"] == "pipeline_step" else t["color"]
rgb = tuple(int(x) for x in c.split(",")) if c else None