I have a draw document. Is there a way to do a global color change? That is, I want to replace everything with color 0x42f320 with 0x20f342.
Is this possible in 1 fell swoop, or do I need lots of middlin swoops?
Everything? Lines? Areas? Texts too?
Per page (slide) you can select all the shapes (Ctrl+A e.g.) and change attributes for all of them in one go (even if grouped) using the known formatting tools. You need to do it for lines, areas, and texts (characters) separately.
There are no means to find all the elements or sub-elements having a specific color, and to restrict the changes to them…
… except …
… by user code.
With the canvas/paintbrush/pigment mindset of simple drawing apps, the dataset is a single pixel map. It is then a matter of examining each pixel’s color and make the swap whenever appropriate.
With the baseboard/object/overlay mindset of Draw and similar vector oriented drawing packages, you have to think a bit differently, as Lupp also indicated. You don’t work on every pixel, but every component of every object. Those objects can receive color by direct formatting, applied style or parent style. You can then change color by altering style settings, or by applying direct formatting.
There is the case of transparency, which will blend color with background/overlapping object. Do you need to handle those blend areas’ final, rendered color? That is not a trivial task.
I am not saying it can’t be done, only suggesting that you give us some context. What do you have and what are you trying to achieve? There may be a better strategy.
Globally, LibreOffice applications encourage the use of styles. A style is a set of formatting metadata attached to a managed object (text in Writer, cell in Calc, drawing objects in Draw, blocks in Impress). This allows separation between content and appearance.
Styles have not the same degree of maturity across all applications but those in Draw are quite decent.
Provided you designed your drawing with style markup to distinguish the various “roles” of your objects (the semantic purpose of the object), you can achieve your requirement through style modification.
For example, styles usually describe the background and border colours separately along with the widths of the line (border).
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My drawing is a bunch of colored rectangles, different colors for different groups. One group has merged with another, so I need to change a bunch of boxes’ colors.
I don’t care about lines/connectors/text, that’s all black anyway.
I wasn’t aware styles existed, I’ll use them next time I make a drawing.
Since your “groups” seem to have a particular significance for you, every “significance” should have its own style. Then you apply this style to the relevant objects and they acquire all the properties of the group. This is quite similar to the usage of styles in Writer.
Did I mention “I didn’t know styles existed”, and “Next drawing will use styles”? I’m guessing it will be a lot faster (actually, was) changing a bunch of boxes than making a whole new drawing?
I read your remark. I just added a principle of use for the community.
If you think my answer addressed correctly your question, be kind enough to tick the gray check mark at left of answer. Do this only once, even if seemingly nothing happens.
Not seeing a gray check mark anywhere. Sorry I was snide a couple days ago, I’d just finished manually changing a bunch of boxes’ colors and, well, was feeling snarky.
Check the mark (
→ 
) to the left of the answer that solves your question.
Not possible. Ajittoz’s reply to use style sheets next time is the solution.