Contrast is subjective and if the cop doesn't like them, that's his call. TPW&D website also shows them to be attached to the hull and your do not appear to be. I can understand why he would not like black on black.
Placement of TX Number and Decal on Vessels:
TX # and validation decal to be placed each side of the vessel, near the bow, in such a position as to provide easy identification.
The number must read from left to right, be in block characters of good proportion not less than three inches in height and be of a color that contrasts with the color of the vessel.
The numerals must be separated from the prefix and suffix by hyphens or equivalent spaces such as the following example: TX 0001 GG or TX-0001-GG.
The validation decal must be affixed in a line and within three (3) inches towards the rear of the boat from the registration number (an example is shown on the decal document).
Vessels registered as antique boats are permitted to display the registration decal on the left portion of the windshield. In the absence of a windshield, the registration decal must be attached to the certificate of number and made available for inspection when the boat is operated on public water.
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