It is narrated from Sayyidah Aishah (may Allah be pleased with her) that when a woman sees blood, she should refrain from prayer until she sees it white like chalk. When she sees that, she should perform ghusl and pray. If after that she sees yellow or brownish discharge, she should perform ablution and pray; and if she sees red blood, she should perform ghusl and pray.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1599