Conspicuity.

CFR · 2024-01-01 · US · Title 14

The exterior surface of the envelope must be of a contrasting color or colors so that it will be conspicuous during operation. However, multicolored banners or streamers are acceptable if it can be shown that they are large enough, and there are enough of them of contrasting color, to make the balloon conspicuous during flight.

official document · source cfr-2024 · id 14:§ 31.83