Welcome to the community! Are you working with a text layer/graphics template or captions? A white text layer with Rec. 709 color space in an HDR sequence (Rec. 2100 HLG or Rec. 2100 PQ) can appear grey because of the relatively lower max brightness levels of Rec. 709 color space. If you are working with a text layer/graphics template in an HDR sequence, you may try these steps & check if they help.
- Select the text layer in the video track
- Navigate to Lumetri Color > Settings tab > Source Clip
- Set Override Media Color Space to Rec. 2100 HLG or Rec. 2100 PQ
In case you are working with captions or in an SDR environment where you don't need HDR, you may set Sequence Color Setup to Direct Rec. 709 (SDR) or Wide Gamut (Tone Mapped) (under Lumetri Color > Settings tab > Sequence).
Hope it helps. Let us know if you have any questions.