PDF/A requires an output intent with an ICC profile if a device color space is used and the respective Default color space is not set (cf. PDF 1.7, 8.6.5.6). Typst writes device color spaces in the following situations:

• When using a CMYK color
• When writing an image with an alpha mask (DeviceGray)

For alpha masks, we should do the right thing. (Is there such a thing as an alpha image profile? Is gamma applied here?)

For CMYK, things are more complicated:

Quick fix:
Fail PDF export if PDF/A compliance is requested and we find DeviceCMYK colors.

Principled solution:
CMYK colors have little meaning without an output condition. We should allow CMYK output colors in PDF/A only if an output intent is set. This output intent should be configurable in the source text, as the CMYK values without it are ambiguous. A color management with a library like qcms is needed to provide the right conversions for RGB in the SVG exporter. This would also improve color accuracy for CMYK in the preview.