The lining (endometrium) of the uterus is red-pink, as you can see from endoscopic photos of the uterus here.
The tissue of endometriosis lesions is not the same but similar to the lining of the uterus and can be red or white, according to the study: Using Location, Color, Size, and Depth to Characterize and Identify Endometriosis Lesions in a Cohort of 133 Women (Fertility and Sterility, 2008):
357 of 544 lesions felt to be endometriosis by the surgeon had
positive histology. Mixed color lesions most commonly contained
endometriosis (76%), with the percent of positive lesions being
similar between single color groups. 58% (164/283) of subtle (red or
white) lesions were positive for endometriosis. Thirty women had only
red or white lesions, and 18 (60%) had at least one lesion positive
Endometriosis lesions can trigger the development of adhesions and internal scars, which are made of fibrous tissue, which is white. According to Mayo Clinic:
Surrounding tissue can become irritated,
eventually developing scar tissue and adhesions — abnormal bands of
fibrous tissue that can cause pelvic tissues and organs to stick to
The page on drseckin website you linked contains images of endometriosis with white tissue, which is, according to the text over the images, due to scarring or adhesions, so not from the endometriosis lesions themselves.