(MYSQL n00b)

I have 3 tables:

id = int(10), photo_id = bigint(20)

PHOTO records limited to 3 million

PHOTO:
+-------+-----------------+
| id    | photo_num       |
+-------+-----------------+
| 1     | 123456789123    |
| 2     | 987654321987    |
| 3     | 5432167894321   |
+-------+-----------------+

COLOR:
+-------+-----------------+---------+
| id    | photo_num       | color   |
+-------+-----------------+---------+
| 1     | 123456789123    | red     |
| 2     | 987654321987    | blue    |
| 3     | 5432167894321   | green   |
+-------+-----------------+---------+


SIZE:
+-------+-----------------+---------+
| id    | photo_num       | size    |
+-------+-----------------+---------+
| 1     | 123456789123    | large   |
| 2     | 987654321987    | small   |
| 3     | 5432167894321   | medium  |
+-------+-----------------+---------+

Both COLOR and SIZE tables will have several million records.

Q1: Is it better to change photo_num on COLOR and SIZE to int(10) and point it to PHOTO's id?

Right now I use these: (PHOTO is no where in the picture)

SELECT * from COLOR WHERE photo_num='xxx';
SELECT * from SIZE WHERE photo_num='xxx';

Q2: How will the SELECT query look if PHOTO id was used in COLOR, SIZE?