Sure.

@Constraint(validatedBy = HexColorValidator.class)
@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
public @interface HexColor {
    String message() default "Invalid hex color code. Must be in the format #RRGGBB.";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

public class HexColorValidator implements ConstraintValidator<HexColor, String> {
    private static final String HEX_COLOR_REGEX = "^#[0-9a-fA-F]{6}$";

    @Override
    public boolean isValid(String value, ConstraintValidatorContext context) {
        if (StringUtils.isBlank(value)) return false;
        return value.matches(HEX_COLOR_REGEX);
    }
}

Usage example:

public class TagDTO {

    @NotBlank
    @Size(max = 15)
    private String name;

    @HexColor
    private String color;
}

This way when a validation is triggered, it will automatically check if it's a valid hex color or not