Thank you for reaching out to us.
We appreciate the sensitive nature of this question.
Disposable undergarments such as you describe are made of synthetic materials and are meant for a single use. The relevant halachot are akin to those of a menstrual pad. (Learn more here.) There is extra room for leniency because of pregnancy and because mixing with urine can amplify impressions of a stain’s quantity.
For the leniencies of stains to apply, this type of undergarment does not need to be colored, but you are correct that using a colored product would be preferable if available.
Urinary incontinence during pregnancy is common and can be inconvenient or embarrassing. Fortunately, products like these can make a positive difference. Additionally, pelvic floor physiotherapy can prove effective even during pregnancy in limiting leakage. If you haven’t yet done so, we would encourage you to seek out this avenue as well.
Please be in touch with any further questions. B’sha’ah tovah!